1 


# 


agricultural  Science  Series 

L.  H.  BAILEY,  Editor 


THE  NATURE  AND 
PROPERTIES  OF  SOILS 


AGRICULTURAL  SCIENCE  SERIES 

UNDER  THE  EDITORSHIP  OF 
L.  H.  BAILEY 


THE  NATURE  AND  PROPERTIES  OF   SOILS, 
by  T.  Lyttleton  Lyon  and  Harry  0.  Buckman 


6. 


THE    NATURE    AND 
PROPERTIES    OF    SOILS 

A  COLLEGE  TEXT  OF  EDAPHOLOGY 


T.  LYTTLETON  LYON 

PROFESSOR    OF    SOIL    TECHNOLOGY,  .CORNELL    UNIVERSITY 


HARRY  O.  BUCKMAN 

PROFESSOR    OF    SOIL    TECHNOLOGY,    CORNELL    UNIVERSITY 


iSeto  got* 

THE  MACMILLAN  COMPANY 

1922 

All  rights  reserved 


PRINTED  IN   THE   UNITED   STATES   OF  AMERICA 


Copyright,  1922, 
By  THE  MACMILLAN  COMPANY. 

Set  up  and  electrotyped.     Published  April,    1922. 


«*. 


TABLE  OF  CONTENTS 

CHAPTER  FAGS 

I.    Some  Conceptions  op  the  Soil  and  Its  Relation  to 

/^              Plants         1 

*\JX-   Soil    Forming    Processes 16 

*^III.   The  Geological  Classification  op  Soils 38 

IV,    The  Soil  Particle  and  Certain  Important  Relations  66 

V.    The  Organic  Matter  op  the  Soil 99 

VI.    The  Colloidal  Matter  op  the  Soil 127 

•*vll.   Soil  Structure  and  Its  Modification 139 

lafSTfT  The  Forms  op  Soil  Water  and  Their  Characteristics  151 

IX.    The  Water  of  the  Soil  in  Its  Relation  to  Plants     .  184 

\XT  The  Control  op  Soil  Moisture 202 

XI.   Soil   Heat 223 

XII.    Soil  Air 247 

XIII.  The  Absorptive  Properties  o*  Soils 263 

XIV.  The   Soil   Solution 275 

XV.    The  Removal  op  Nutrients  prom  the  Soil  by  Cropping 

and  Leaching 289 

^■XVl.   Chemical  Analysis  op  Soils 311 

VKVII.   Alkali    Soils 328 

V^VIII.    Soil    Acidity 345 

XIX.   Liming  the  Soil 362 

«-   XX.    Soil  Organisms,  Carbon,  Sulfur  and  Mineral  Cycles  384 

sJLxxi.    Soil    Organisms — the   Nitrogen    Cycle 409 

XXII.    Commercial    Fertilizer    Materials 442 

XXIII.  The  Principles  of  Fertilizer  Practice 471 

XXIV.  Farm   Manure t 499 

XXV.    Green  Manure 535 

XXVI.    The  Maintenance  of  Soil  Fertility 552 

Index  of  Authors 561 

Index  of  Subject  Matter 567 

v 


NATURE  AND  PROPERTIES 
OF  SOILS 

CHAPTER  I 

SOME  CONCEPTIONS  OF  THE  SOIL  AND  ITS  RELA- 
TIONS TO  PLANTS 

Due  to  the  action  of  climatic  agencies  the  outer  solid  por- 
tions of  the  earth  readily  pass  into  a  loose  and  disintegrated 
condition.  This  layer,  although  superficial  and  insignifi- 
cant in  comparison  to  the  bulk  of  the  earth,  has  performed 
and  is  still  performing  a  marvelous  function.  Life  on  the 
earth  has  been  slowly  but  steadily  developing  and  changing 
until  we  see  about  us  the  forms  that  characterize  our  age. 
This  evolution  has  depended  to  no  small  degree  on  this  super- 
ficial layer  of  decomposed  rock  with  its  admixture  of  de- 
caying organic  matter  which  together  form  the  soil.  In 
this  medium  many  and  varied  organisms  have  lived  and  from 
it  have  drawn,  wholly  or  in  part,  their  sustenance,  leaving 
as  a  recompense  a  contribution  of  organic  debris,  which  in  its 
turn  has  given  rise  to  reactions  of  almost  unbelievable  com- 
plexity. 

Like  the  life  which  it  has  sustained  and  nourished,  the 
soil  has  been  changing  and  evolving.  The  soil  of  today 
is  not  the  soil  of  yesterday  nor  will  it  be  the  soil  of  tomorrow. 
It  is  never  still.  It  is  continually  seeking  a  mechanical  and 
chemical  adjustment  with  the  forces  which  surround  it  or 

1 


2     NATURE  AND  PROPERTIES  OF  SOILS 

are  active  within  its  precincts.  Such  an  equilibrium  it  never 
attains  and  thus  the  evolution  goes  on  and  on.  It  is  this 
continual  change  and  this  endless  response  to  environment 
that  makes  the  soil  useful  to  plants.  The  disintegrating 
rock  and  the  decaying  organic  additions  are  thus  converted 
into  a  mechanical  support  for  plants,  while  at  the  same 
time  they  are  forced  to  liberate  the  nutrients  essential  to 
plant  growth. 

In  the  light  of  its  origin  and  function  the  soil  may  be 
denned  as  a  mixture  of  broken  and  weathered  fragments  of 
rock  and  decaying  organic  matter,  which  covers  the  earth 
in  a  thin  layer  and  supplies  mechanical  support  and  in  part 
sustenance  to  plants. 

This  debris  of  rock  and  plant  residue,  teeming  with  its 
microscopic  life  and  ever  restless  in  its  endless  efforts  at 
equilibrium,  is  the  arable  soil  from  which  man  must  obtain 
his  bread.  As  the  light  of  investigation  is  thrown  on  it, 
new  changes,  new  functions  and  new  and  unsuspected  re- 
lationships are  brought  to  view  until  the  story  of  the  soil 
may  be  retold  with  a  clearer  insight  into  those  processes 
that  render  it  useful  to  man. 

1.  Composition  of  the  soil. — The  soil  as  denned  is  com- 
posed of  two  general  classes  of  material,  mineral  and  organic. 
The  former  in  most  cases  makes  up  from  90  to  99  per 
cent,  by  weight  of  the  dry  substance  of  a  soil,  the  organic 
matter,  except  in  the  case  of  peat  and  muck,  being  in  rela- 
tively smaller  amounts.  In  spite  of  the  low  proportion  of 
organic  matter  its  presence  is  vital,  not  only  because  of  its 
influence  physically  but  because  of  the  nutrients,  especially 
nitrogen,  that  it  carries.  The  mineral  portion  of  a  soil 
functions  as  a  frame-work  and  as  a  source  of  certain  chem- 
ical elements,  which  are  necessary  to  proper  crop  growth  and 
development. 

It  must  be  realized  at  the  very  outset  that  the  two  main 
constituents  in  a  normal  soil  exist  in  very  intimate  relation- 


SOME  CONCEPTIONS  OF  THE  SOIL 


ship,  reactions  occurring  not  only  within  each  group  but 
between  the  groups  as  well.  Unless  such  interactions  take 
place  it  is  unlikely  that  the  mixture  will  ever  be  in  a  con- 
dition either  chemically,  physically  or  biologically  to  sus- 
tain plant  growth.  These  reactions,  although  very  complex, 
take  place  with  surprising  ease  and  rapidity.  As  a  con- 
sequence the  study  of  this  complex,  heterogeneous  and 
highly  dynamic  mass  that 
we  call  the  soil  is  often  be- 
set with  difficulties  that 
completely  baffle  our  pres- 
ent facilities  for  its  study. 
2.  Soil-forming  rocks.1 
— In  any  study  of  soil 
origin  or  composition,  how- 
ever cursory,  the  geological 
phases  of  the  problem  im- 
mediately force  attention. 
This  is  due  to  the  bearing 
that  certain  geological  phe- 
nomena have  on  soil  condi- 
tions and  crop  growth.  In 
the  soil  we  find  that  the 
inorganic  materials  have 
originated  from  the  com- 
mon rocks.  The  best  known 
country  rocks  are  of  course 
involved  because  they  present  the  greatest  outcrop  surface  and 
of  necessity  must  contribute  most  to  the  mineral  fabrication  of 
the  soil.  They  are  classified  under  three  heads — igneous,  sedi- 
mentary and  metamorphic.  The  most  important  types  from 
the  standpoint  of  soil  formation  are  the  following : 

1For  excellent  non-technical  discussions  of  rocks  and  minerals: — 
Pirsson,  L.  V.,  BocJcs  and  Bock  Minerals;  New  York,  1915.  Merrill, 
G.  P.,  Rocks,  Bock  Weathering  and  Soils;  New  York,  1906. 


ORGANIC- 
10% 

Fig.  1.  —  Volume  composition  of  a 
loam  soil  when  in  good  condition 
for  plant  growth.  The  air  and 
water  in  a  soil  are  variable  and 
their  proportion  determines  to  a 
considerable  degree  the  productiv- 
ity. 


NATURE  AND  PROPERTIES  OF  SOILS 


Igneous 

Sedimentary 

Metamorphic 

Granite 

Limestone 

Marble 

Syenite 

Dolomite 

Schist 

Diorite 

Shale 

Slate 

Gabbro 

Sandstone 

Quartzite 

Basalt 

Conglomerate 

Gneiss 

The  mineralogical  complexity  of  rocks  has  an  important 
bearing  on  the  question  of  soil  formation  and  soil  composi- 
tion. The  fragments  of  any  soil  are,  for  the  most  part,  dis- 
tinguishable as  separate  minerals  rather  than  as  mineral  aggre- 
gates. For  example,  a  soil  from  a  granite  would  be  char- 
acterized by  separate  grains  of  quartz,  orthoclase,  micro- 
cline  and  perhaps  mica  rather  than  by  fragments  of  the  orig- 
inal granite  itself.  Again,  it  is  the  composition  of  the  easily 
decomposable  minerals  rather  than  the  composition  of  the 
bulk  rock  that  determines  what  simplifications  shall  occur, 
what  new  substances  shall  arise  in  the  soil  and  what  elements 
shall  be  liberated  for  plant  use. 

3.  Soil  minerals. — Although  hundreds  of  minerals  have 
been  identified,  comparatively  few  are  common  or  important x 
in  rock  formation.  As  a  consequence,  the  list  of  im- 
portant minerals  found  in  soils  will  be  correspondingly  cur- 
tailed, although  enough  are  always  present,  especially  in  the 
finer  portions,  to  make  the  soil  very  complex  mineralogically. 
The  minerals  as  to  origin  may  be  divided  into  two  groups: 
(1)  those  that  persist  from  the  original  rock  and  (2)  those 
that  are  produced  by  the  decomposition  of  the  original  min- 
erals, during  soil  formation.    For  example,  the  quartz  grains 

irThe  following  table  indicates  the  approximate  proportions  of  the 
common  minerals  in  the  earth's  crush  to  a  depth  of  ten  miles: 

Feldspars   57.8%    Clay 1.0% 

Amphibole  and  Py-  Carbonates   5 

roxene   16.0       Limonite 2 

Quartz     12.7       All  others 8.2 

Mica    3.6 

Recalculated  from  Clarke,  F.  W.,  Data  of  Geochemistry ;  U.  S.  Geol. 
Survey,  Bui.  695,  pp.  32-33.    1920. 


SOME  CONCEPTIONS  OF  THE  SOIL 


of  soil  almost  always  come  directly  from  the  original  rock 
as  do  particles  of  orthoclase,  biotite,  and  apatite.  Hematite, 
the  kaolinite  group  and  the  chlorite  and  epidote  groups 
generally  originate  in  soils  through  weathering.  The  fol- 
lowing list  of  minerals  is  by  no  means  complete,  yet  it  includes 
the  more  important  forms  from  the  soil  and  plant  standpoint. 

A  LIST  OF  THE  MOST  IMPORTANT  SOIL  MINERALS.1 
(The  elements  in  bold  type  are  those  necessary  for  plant  nutrition.) 


1.  Quartz 

Si02 

2.  Orthoclase  and 

KAlSi308 

Microcline  feldspar 

3.  Muscovite  mica 

KH2Al3Si3012 

4.  Biotite  mica 

KHMgFeAl2Si3012 

5.  Plagioclase  feldspar 

Ca  and  Na  aluminum  silicates 

6.  Calcite  and  Dolomite 

CaC03  and  (Ca,  Mg)  C03 

7.  Hornblende  and  Augite 

Ca,  Mg,  Fe  aluminum  silicates 

8.  Olivine 

(Mg,  Fe)2Si04 

9.  Apatite 

Ca5  (P04)3(C1,F) 

10.  Kaolinite  group 

Typified  by  kaolinite. 

H4Al2Si209 

11.  Serpentine  and  Talc 

Hydrated  Mg  silicates 

12.  Chlorite  group 

Hydrated  Mg,  Fe  aluminum 

silicates 

13.  Epidote  group 

Hydrated  Ca,  Fe  aluminum 

silicates 

14.  Hematite 

Fe203 

15.  Limonite  group 

Typified  by  limonite  2  Fe203. 

3  H20 

1  Below  are  some  of  the  most  important  mineralogical  investigations  of 
soil:  McCaughey,  W.  G.,  and  Williams,  H.  F.,  The  Microscopic  De- 
termination of  Soil-Forming  Minerals;  U.  S.  Dept.  Agr.,  Bur.  Soils,  Bui. 
91.  1913.  Plummer,  J.  K.,  Petrography  of  Some  North  Carolina 
Soils  and  Its  Relationship  to  their  Fertilizer  Requirements,  Jour.  Agr. 
Res.,  Vol.  V,  No.  13,  pp.  569-581.  1915.  Robinson,  W.  O.,  The  Inor- 
ganic Composition  of  Some  Important  American  Soils;  U.  S.  Dept.  Agr., 
Bui.  122.     Aug.,  1914. 


6     NATURE  AND  PROPERTIES  OF  SOILS 

4.  Importance  of  soil  minerals. — Quartz  is  found  in  al- 
most all  soils,  making  up  often  from  80  to  90  per  cent,  of 
the  composition,  although  a  range  from  40  to  70  per  cent, 
is  more  common.  Its  universal  presence  is  due  to  its  hard- 
ness and  insolubility.  Quartz  is  a  make-weight  material, 
however,  as  it  probably  contributes  but  little  to  plant  nutri- 
tion. In  the  form  of  sand,  quartz  has  a  great  influence  on 
the  friability  of  soil,  improving  and  maintaining  the  phys- 
ical condition  to  a  marked  degree. 

Orthoclase,  microcline,  muscovite  and,  to  a  lesser  degree, 
biotite  are  important  because  of  their  potash  content.1  They 
decompose,  often  rather  readily,  into  kaolinite  and  similar 
products,  thus  liberating  potassium  in  soluble  form.  The 
plagioclase  feldspars  also  give  rise  to  kaolinite.  They  carry, 
however,  sodium  and  calcium.  The  latter  element2  plays  an 
important  role  in  soil  both  as  a  nutrient  and  as  an  amend- 
ment. When  not  sufficiently  active  it  must  be  applied  in 
some  form.  Calcite  and  dolomite  also  carry  calcium.  Horn- 
blende and  augite  bear  calcium  as  well  as  magnesium  and 
iron.  Olivine  is  a  magnesium  and  iron  silicate.  The  oxida- 
tion of  the  iron  of  the  above  minerals  gives  rise  to  hematite, 
so  common  as  a  red  coloring  matter  of  soil. 

Practically  all  of  the  phosphorus  of  the  soil,  either  organic 
or  inorganic,  has  its  origin  in  apatite,  yet  this  mineral  occurs 
but  sparingly  either  in  rock  or  soil.  It  makes  up  but  6  per 
cent,  of  igneous  rocks.  This  accounts  for  the  small  percent- 
age of  phosphoric  acid  in  most  soils  and  explains  why  it  is 
often  added  in  fertilizers.3 

1Plummer,  J.  K.,  Availability  of  Potash  in  Some  Common  Soil- 
forming  Minerals,  Jour.  Agr.  Bes.,  Vol.  XIV,  No.  8,  pp.  297-315. 
Aug.,  1918.  de  Turk,  E.,  Potassium-bearing  Minerals  as  a  Source  of 
Potassium  for  Plant  Growth;  Soil  Sci.,  Vol.  8,  No.  4,  pp.  269-301.     1919. 

aShorey,  E.  C.  et  al.,  Calcium  Compounds  in  Soils;  Jour.  Agr.  Ees., 
Vol.  VII,  No.  3,  pp.  57-77.    Jan.,  1917. 

8  Fry,  W.  H.,  Condition  of  Phosphoric  Acid  Insoluble  in  Hydro- 
chloric Acid;  Jour.  Ind.  and  Eng.  Chem.,  Vol.  V,  No.  8,  pp.  664- 
665.     1913. 


SOME  CONCEPTIONS  OF  THE  SOIL 


The  members  of  the  kaolinite  group  are  decomposition  prod- 
ucts resulting  from  the  .dgcay  of  the  feJdnpars  and  similar 
minerals.  While  kaolinite  itself  shows  no  nutrients  in  its 
formula,  it  often  carries  considerable  calcium,  potassium, 
magnesium  and  phosphorus  by  absorption.  Moreover,  its 
close  association  with  other  decomposition  products  such  as 
serpentine,  talc,  chlorite  and  epidote  tends  to  accentuate  its 
importance  in  plant  nutrition.  The  plasticity  and  cohesion 
imparted  to  a  soil  by  the  presence  of  the  kaolinite  group 
and  its  associated  minerals  are  of  great  practical  importance 
as  is  also  the  capacity  to  hold,  either  physically  or  chemically, 
the  bases  already  mentioned. 

Hematite  and  limonite  are  simple  iron  compounds  and 
usually  occur  in  the  soil  as  a  result  of  the  decomposition  of 
certain  iron-bearing  minerals  such  as  biotite,  hornblende  and 
augite.  These  iron  compounds  impart  the  red  and  yellow 
colors  so  characteristic  of  certain  southern  soils.  Most  of  the 
soluble  iron  of  the  soil  has  its  source  in  these  minerals.  Hema- 
tite and  limonite  are  produced  by  the  same  general  processes 
as  are  the  kaolinite  group  and  are  found  in  very  intimate 
contact  with  the  serpentine,  epidote,  chlorite  and  kaolinite. 

5.  Soil  organic  matter. — One  of  the  essential  differences 
between  a  normal  fertile  soil  and  a  mass  of  rock  fragments 
lies  in  the  organic  content  of  the  former.  The  organic  matter 
practically  all  comes  from  plants  and  animals  that  have  in- 
vested the  surface  of  the  soil  and  the  soil  material.  Through 
the  agency  of  bacteria  and  other  organisms  with  which  the 
soil  is  liberally  supplied,  this  organic  tissue  quickly  loses  its 
original  form,  and  becomes  the  dark  incoherent  material  so 
noticeable  in  fertile  soils.  The  decay  is  not  one  of  immediate 
simplification,  as  might  be  supposed.  The  split-off  compounds 
react  not  only  with  materials  of  a  similar  origin  but  also 
with  the  decomposing  mineral  fragments.  This  tendency  pro- 
vides the  intimate  relationship  between  the  organic  and  in- 
organic constituents  of  the  soil  already  emphasized  as  an  ex- 


8     NATURE  AND  PROPERTIES  OF  SOILS 

ceedingly  desirable  condition.  Incidentally  the  soil  is  ren- 
dered thereby  very  much  more  difficult  to  study,  especially 
chemically. 

The  incorporation  of  organic  matter  in  any  soil,  either  by 
natural  or  artificial  means,  tends,  if  the  proper  decay  occurs, 
to  make  the  soil  more  friable.  The  water  capacity  is  markedly 
increased  and  the  vigor  of  the  bacterial  and  chemical  activ- 
ities stimulated  to  a  marked  degree.  As  these  two  latter 
actions  progress,  some  of  the  organic  matter  passes  into  simple 
combinations,  allowing  certain  elements  to  become  available 
to  crops.  Nitrogen,  which  is  held  in  the  soil  largely  in  organic 
combination,  emerges  in  the  form  of  ammonia,  nitrites  and 
nitrates.  It  is  from  a  salt  of  nitric  acid  that  most  plants 
absorb  their  nitrogen.  Small  amounts  of  sulfur,  phosphorus, 
potassium  and  calcium  are  liberated  from  the  tissue  as  decay 
proceeds.  The  largest  product  of  organic  decay,  however,  is 
carbon  dioxide  (C02),  which  in  the  soil  becomes  important 
as  a  solvent  for  minerals,  thus  hastening  the  decomposition 
processes. 

6.  Factors  for  plant  growth. — The  growth  and  develop- 
ment of  a  plant  depends  on  two  sets  of  factors,  the  internal 
and  external.  The  latter  may  be  classified  as  follows:  (1) 
mechanical  support,  (2)  heat,  (3)  light,  (4)  oxygen,  (5) 
water,  and  (6)  nutrients.1  With  the  exception  of  light,  the 
soil  supplies,  either  wholly  or  in  part,  all  of  these  conditions. 
Mechanical  support  is  a  function  entirely  of  the  soil.  The 
comparatively  loose  and  friable  condition  presented  by  most 
soils  allows  ample  foothold  to  the  ramifying  roots. 

Air  and  water  are  easily  supplied  because  of  the  open 
condition  of  the  soil,  and  its  large  pore  spaces.  Temperature 
depends  almost  wholly  on  climatic  relationships.     The  water 

1  Nutrients  are  materials  from  which  food  may  be  elaborated  once 
they  have  been  absorbed  by  plants.  The  energy  for  this  synthetic  proc- 
ess comes  from  the  sun.  A  food  is  any  substance  from  which  the  plant 
may  obtain  energy  for  its  normal  processes.  A  large  proportion  of  the 
materials  absorbed  ty  plants  are  nutrients. 


SOME  CONCEPTIONS  OF  THE  SOIL  9 

of  the  soil  acts  as  a  plant  nutrient  in  itself  and  functions 
also  as  a  solvent  for  other  materials.  By  its  circulation  it 
not  only  promotes  solution  but  it  continually  brings  nutrient 
elements  in  contact  with  the  absorbing  surfaces  of  the  roots. 
The  two  prime  functions  of  the  soil  are  thus  realized  through 
the  factors  discussed  above — mechanical  support  and  a  suffi- 
cient supply  of  certain  nutrient  elements  under  favorable 
conditions. 

7.  Nutrient  elements.1 — Although  the  physical  condition 
of  the  soil  exerts  a  far-reaching  influence  on  plant  growth, 
the  relationships  involved  are  more  readily  understood  than 
those  which  have  to  do  with  plant  nutrition.  Moreover,  the 
solubility  of  the  necessary  nutrients  is  very  closely  related 
to  the  complex  processes  of  soil  formation.  Ten  elements2 
are  usually  considered  as  necessary  for  plant  growth.  If  one 
is  lacking,  normal  development  will  not  occur.  They  may 
be  classified  as  follow^ :• 

From  air  or  water  From  the  soil 

Carbon  Nitrogen  Calcium 

Oxygen  Phosphorus  Magnesium 

Hydrogen  Potassium ,  Sulfur 

Nitrogen  Iron 

Plants  obtain  most  of  their  carbon  and  oxygen  directly 
from  the  air  by  photosynthesis  and  respiration.  The  hydro- 
gen comes,  at  least  partially,  from  water.  All  of  the  other 
elements,  except  a  small  amount  of  nitrogen  utilized  directly 
from  the  air  by  certain  plants,  are  obtained  from  the  soil. 
It  must  not  be  inferred,  however,  that  the  bulk  of  the  plant 

1  For  an  excellent  discussion  of  the  functions  of  plant  nutrients,  see 
Russell,  E.  J.,  Soil  Conditions  and  Plant  Growth,  Chap.  II,  pp.  30-46; 
New  York.    1915. 

*  It  may  be  possible  that  manganese  and  silicon  and  possibly  chlorine 
and  fluorine  function  as  nutrients.  They  as  well  as  sodium,  aluminum, 
titanium,  barium,  strontium,  and  certain  rarer  elements  are  found  in 
plant  ash. 


10  NATURE  AND  PROPERTIES  OF  SOILS 

tissue  is  fabricated  from  the  soil.  Quite  the  reverse  is  true. 
Fresh  plant  tissue  generally  carries  only  from  .5  to  2.5  per 
cent,  of  mineral  material.  In  spite  of  this,  it  is  the  mineral 
elements  of  nutrition  that  generally  limit  crop  growth  since 
a  plant  can  always  obtain,  except  in  cases  of  drought  or 
disease,  unlimited  amounts  of  carbon,  hydrogen  and  oxygen. 

8.  Primary  nutrient  elements. — While  all  of  the  seven 
soil  nutrients  must  be  available  that  plants  may  grow  normally, 
only  four  or  five  are  likely  to  become  limiting  factors.  The 
others  are  almost  always  in  great  sufficiency.  These  few, 
nitrogen,  phosphorus,  potassium,  calcium  and  occasionally 
sulfur,  receive  as  a  consequence  especial  attention.  They 
may  limit  growth  because  they  are  actually  lacking  or  be- 
cause their  availability  is  low.  These  conditions  often  occur 
in  the  same  soil. 

Combined  nitrogen  exists  in  the  soil  to  a  large  degree  as 
a  part  of  the  partially  decayed  organic  matter  present 
therein.1  As  decay  proceeds,  small  quantities  of  this  nitrogen 
appear  as  ammonia  in  combination  with  some  acid  radical 
such  as  the  chloride  or  sulfate  or  with  the  hydroxal  group. 
Later,  it  is  changed  through  further  bacterial  action  to  the 
nitrate  form,  united  with  some  bases  such  as  calcium  or  po- 
tassium. It  is  from  this  latter  combination  that  most  plants 
obtain  the  greater  part  of  their  nitrogen.  These  inorganic 
nitrogen  compounds,  present  at  any  one  time  in  a  soil,  are 
but  a  small  proportion  of  the  total  soil  nitrogen.  The  air 
both  above  the  soil  and  that  circulating  within  its  pores  has 
been  the  original  source  of  all  the  combined  nitrogen.  Nat- 
ural processes  have  facilitated  the  combination  which  has  been 
necessary  for  such  a  transfer.     The  encouragement  of  such 

1  Certain  rocks,  particularly  those  of  a  sedimentary  nature,  carry 
considerable  nitrogen.  When  such  rocks  weather,  this  nitrogen  tends 
to  become  available.  The  organic  matter,  therefore,  does  not  absolutely 
control  the  amount  of  nitrogen  in  a  soil.  Hall,  A.  D.,  and  Miller, 
N.  H.  J.,  The  Nitrogen  Compounds  of  the  Fundamental  Bocks;  Jour. 
Agri.  Sci.,  Vol.  II,  Part  4,  pp.  343-345.     July,  1908. 


SOME  CONCEPTIONS  OF  THE  SOIL  11 

fixation  processes,  especially  those  of  a  biological  nature,  is 
a  feature  of  practical  soil  improvement. 

Phosphorus  has  its  origin  in  the  mineral  apatite  (Ca5- 
(P04)8(C1,F))  and  exists  in  the  soil  not  only  in  this  form 
but  as  tri-calcium  phosphate  (Ca3(P04)2),  iron  and  alum- 
inum phosphates  (FeP04  and  A1POJ  and  in  certain  other 
inorganic  complexes.  It  also  exists  in  organic  combinations 
of  a  constantly  varying  nature.  It  probably  is  utilized  by  the 
plant  as  a  simple  phosphate  such  as  the  mono-  or  di-calcium 
salt  (CaH4(P04)2  and  Ca2H2(P04)2). 

Potassium,  as  already  stated,  occurs  in  the  soil  in  orthoclase 
and  microcline  (KAlSi308),  in  mica,  especially  muscovite 
(H2KAl3Si3012),  and  in  other  aluminum  silicates,  both  hy- 
drated  and  non-hydrated.  These  complex  forms  supply  potash 
to  the  soil  solution  and  thus  to  the  plant  at  a  more  or  less 
rapid  rate  in  the  bicarbonate,  carbonate,  chloride,  nitrate,  and 
sulfate  forms. 

Calcium,  while  necessary  in  the  soil  as  a  nutrient,  also 
functions  as  an  amendment  in  that  it  seems  to  preserve  a 
proper  soil  reaction.  It  is  possible  that  this  relationship  is  as 
much  nutritive  as  strictly  chemical.  Calcium  exists  in  the  soil 
in  many  minerals,  of  which  calcite,  plagioclase  feldspar,  horn- 
blende and  augite  are  perhaps  the  most  important.  It  is 
carried  as  an  absorbed  compound  by  kaolinite  and  similar 
materials.  Calcium  becomes  available  in  the  soil  as  the  ni- 
trate, bicarbonate,  chloride,  phosphate,  and  sulfate. 

Sulfur  is  found  in  the  soil  in  rather  small  amounts  and 
generally  forms  a  part  of  the  organic  matter.  Inorganically 
it  usually  occurs  as  a  sulfate  combined  with  the  common 
bases.  In  this  form  it  is  available  to  plants.  The  original 
source1  of  most  of  the  soil  sulfur  has  been  pyrite  (FeS2),  the 

1  Considerable  sulfur  is  brought  to  the  soil  in  atmospheric  precipita- 
tion. From  5  to  150  pounds  an  acre  a  year  have  been  reported.  Wilson, 
B.  D.  Sulfur  Supplied  to  the  Soil  in  Bain  Wacer,  Jour.  Amer.  Soc. 
Agron.,  Vol.  13,  No.  5,  pp.  226-229.     1921. 


12 


NATURE  AND  PROPERTIES  OF  SOILS 


commonest  sulfide  of  this  element.  Although  sulfur  is  no 
more  abundant  in  the  average  soil  than  phosphorus,  it  is 
generally  not  considered  as  an  extremely  important  fertilizing 
constituent. 

It  is  interesting  to  note  at  this  point  the  amounts  of  the 
above  elements  in  ordinary  mineral  soils.    Generally  the  nitro- 


% 


5%       10%      i$% 


Si02 


80.ll 


r«z05+Al205— 9.0l 

Na20 2.0H 

K20 1.5  ■ 

CaO .61 

M9O 5* 

P205 .11 

SO3 .11 

M Z\ 

organic- 


fig.  2. — Chemical  composition  of  a  representative  productive  soil. 


gen  (N)  may  range  from  .1  to  .2  per  cent.,  the  phosphoric 
acid  (expressed  as  P205)  from  .05  to  .30  per  cent,  and  the 
potash  (expressed  as  K20)  from  0.5  to  2.0  per  cent.  Of  the 
plant  nutrients  in  the  soil  nitrogen,  although  usually  present 
in  small  quantities,  is  relatively  more  available  than  is 
phosphoric  acid  or  even  potash.  Phosphoric  acid  may  be  in 
the  minimum  because  of  its  unavailability  as  well  as  because 
of  the  small  quantity.    Potash  is  commonly  present  in  rela- 


SOME  CONCEPTIONS  OF  THE  SOIL  13 

tively  large  amounts.  Its  occurrence  in  complex  and  insoluble 
silicates  makes  its  availability  of  vital  consideration.  The 
presence  of  abundant  organic  matter  may  have  much  to  do 
with  the  liberation  of  sufficient  potash  for  vigorous  plant 
growth. 

The  amount  of  lime  (expressed  as  CaO)  in  soils  is  difficult 
to  state  with  any  degree  of  satisfaction  because  of  a  very 
wide  range  in  composition.  Some  soils  carry  only  a  fraction 
of  a  per  cent.,  while  others,  especially  those  formed  under 
conditions  where  an  originally  high  calcium  content  has  been 
maintained  or  where  calcium  has  accumulated,  show  as  much 
as  10  or  12  per  cent.  The  variability  of  the  sulfur  is  much 
less.  A  range  from  .02  to  .30  per  cent,  of  sulfur  (expressed 
as  S03)  will  include  most  soils. 

It  is  interesting  at  this  point  to  note  the  average  composi- 
tion of  thirty-five  representative  American  surface  soils1, 
which  were  studied  by  the  United  States  Bureau  of  Soils  dur- 
ing a  systematic  investigation  of  the  arable  lands  of  the  United 
States  east  of  the  Rocky  Mountains.  A  comparison  of  these 
data  with  those  setting  forth  the  composition  of  the  litho- 
sphere  2  may  be  made  with  profit.     (Table  I,  page  14.) 

It  is  immediately  noticeable  that  silicon,  aluminum,  and 
iron  make  up  the  greater  portion  of  both  soil  and  lithosphere 
and  that  the  nitrogen,  sulfur  and  phosphorus  are  particu- 
larly low  in  both  cases.  Magnesium,  calcium,  sodium,  and 
potassium  occur  in  fair  amounts,  especially  in  the  earth's 
crust.  It  is  noticeable  also  that  the  soil  is  much  higher  than 
the  lithosphere  in  silicon,  nitrogen,  organic  matter,  and  car- 
bon but  much  lower  in  all  of  the  other  constituents.  These 
differences  have  developed  as  a  result  of  the  losses  and  gains 
during  soil  formation. 

1  Robinson,  W.  O.  et  ah,  Variations  in  the  Chemical  Composition  of 
Soils;  U.  S.  Dept.  Agr.,  Bui.  551.      June,  1917. 

3  The  Lithosphere  refers  to  the  solid  portion  of  the  earth,  in  this  case 
to  a  depth  of  ten  miles.  Clarke,  F.  W.,  Data  of  Geochemistry;  U.  S. 
Geol.  Survey  Bui.  695,  p.  33.     1920. 


14  NATURE  AND  PROPERTIES  OF  SOILS 

Table  I 

Comparison  of  the  Chemical  Composition  op  American 
Surface  Soils  with  that  of  the  Lithosphere. 


Constituents  * 

35  American 

Composition  of 

Surface  Soils 

LITHOSPHERE 

Si02 

84.67 

59.77 

A1203 

6.73 

14.89 

Ti02 

.66 

.77 

Fe203 

2.53 

6.25 

MnO 

.06 

.09 

Na20 

.49 

3.25 

K20 

1.03 

2.98 

CaO 

.40 

4.86 

MgO 

.27 

3.74 

P205 

.09 

.28 

S03 

.09 

.28 

Nitrogen 

.07  a 

— 

Organic  Matter 

2.61b 

— 

Carbon 

1.51c 

.03 

(a)   Average  of  22  soils  only,     (b)  Average  of  13  soils  only, 
(c)    Calculated  from  the  organic  matter. 

9.  The  soil  and  the  plant. — As  the  soil  considered  agri- 
culturally is  essentially  a  medium  for  crop  production,  its 
rational  study  has  to  do  with  the  consideration  and  applica- 
tion of  such  scientific  principles  as  have  a  bearing  on  prac- 
tical soil  management.  Anything  that  makes  clearer  the 
relationships  between  soil  and  crop  has  a  proper  place.  Un- 
less a  scientific  phase  has  a  crop  relation,  either  directly  or 
indirectly,  it  need  receive  but  scant  consideration.  The  com- 
position of  the  soil,  its  chemical  and  biological  changes,  its 
physical  peculiarities  and  its  reaction  to  certain  additions 
must  receive  especial  attention.    More  knowledge  of  the  soil 

1  Soils  contain  many  other  elements,  although  in  small  amounts,  such 
as  chlorine,  barium,  caesium,  chromium,  lithium,  molybdium,  rubidium, 
vanadium,  etc.  Eobinson,  W.  O.,  The  Inorganic  Constituents  of  Some 
Important  American  Soils;  U.  S.  Dept.  Agr.,  Bui.  122.     Aug.,  1914. 


SOME  CONCEPTIONS  OF  THE  SOIL  15 

will  mean  better  systems  of  management  and  will  allow  the 
farmer  to  fulfill  to  a  greater  degree  his  duty  to  himself  and 
to  the  State — the  production  of  paying  crops  and  the  passing 
on  to  the  next  generation  of  a  soil  depleted  as  little  as  possible 
in  fertility. 


CHAPTER  II 
SOIL-FORMING  PROCESSES 

The  forces  which  have  to  do  with  soil  formation  are  largely 
climatic  in  nature.  They  promote  the  physical  and  chemical 
breaking  down  of  rock  masses,  they  intermix  there  with  the 
decaying  organic  matter  and  they  shift  the  products  from 
place  to  place.  Even  after  the  soil  is  apparently  at  rest  and 
has  become  an  effective  agency  in  plant  production,  these 
same  forces  are  still  much  in  evidence.  The  physical  and 
chemical  evolutions  through  which  mineral  and  organic  mate- 
rials at  or  near  the  earth's  surface  are  passing  due  to  natural 
forces  are  spoken  of  as  weathering.1  Erosion  and  deposition 
are  terms  referring  to  the  natural  translocations  which  soils 
and  soil  materials  are  frequently  forced  to  undergo. 

If  a  soil  represents  a  condition  more  stable  than  the  rock, 
the  rock  change  is  in  that  direction.  If  a  soil  presents  con- 
stituents or  conditions  not  wholly  stable  to  the  forces  effective 
at  that  particular  time,  it  in  turn  seeks  a  change  by  an  altera- 
tion or  an  elimination.  A  cycle  of  development  is  thus  set 
up  proceeding  from  youth  to  adolescence  and  even  into  old 
age.  According  to  conditions,  soils  may  age  rapidly  or  slowly. 
Rejuvenation  may  even  occur,  while  cases  of  arrested  develop- 
ment may  exist  for  short  periods. 

10.  Soil-forming  processes  classified. — While  weather- 
ing, with  the  changes  in  form  and  composition  which  inva- 
riably accompany  it,  profoundly  affects  topography,  it  is  very 

1  The  term  weathering  is  somewhat  misleading  since  it  comprehends 
forces  other  than  those  generally  considered  as  weather.  All  of  the 
forces  involved,  however,  depend  upon  climatic  conditions. 

16 


SOIL-FORMING  PROCESSES  17 

superficial  in  comparison  to  the  earth's  bulk.  Nevertheless, 
the  weathered  mantle,  in  spite  of  its  comparative  insignifi- 
cance, presents  an  effective  medium  for  plant  growth.  The 
agencies  of  formation,  therefore,  demand  more  than  the  brief 
mention  just  given.  These  forces  are  geologic  when  the  soil 
is  being  evolved,  but  once  the  soil  materials  are  in  place,  the 
actions  become  localized  and  the  influences  may  be  considered 
as  soil  processes  rather  than  more  broadly  geological. 

The  soil-forming  processes1,  while  diverse  both  in  action 
and  product,  may  be  classified  under  two  heads,  mechanical 
and  chemical.  The  former  is  often  designated  as  disintegra- 
tion, the  latter  as  decomposition. 

SOIL-FORMING  PROCESSES 

I.     Mechanical  (disintegration) 

A.  Erosion  and  deposition. 

Water,  ice  and  wind.2 

B.  Temperature  change. 

Differential  expansion  of  minerals,  exfoliation 
and  frost. 

C.  Biological  influences. 

Plants  and  animals. 
II.     Chemical   (decomposition) 

A.  Oxidation  and  deoxidation. 

B.  Carbonation  and  decarbonation. 

C.  Hydration  and  dehydration. 

D.  Solution. 

11.  The  mechanical  action  of  water. — From  the  time  that 
that  water  as  rain  beats  down  upon  the  solid  earth  until  it 
is  finally  discharged  into  the  ocean,  there  to  pound  as  waves 
upon  the  bordering  lands,  it  is  moving,  sorting,  and  rework- 
ing the  products  of  weathering.     Water  to  erode  must  be 

1  For  a  complete  and  detailed  discussion  of  soil  formation,  see  Merrill, 
G.  P.,  Boclcs,  Bock  Weathering  and  Soils;  New  York.  1906.  Also, 
Emerson,  H.  L.,  Agricultural  Geology;  New  York.    1920. 

2  Gravity  is  generally  included  in  this  group.  While  indirectly  of 
great  significance  in  soil  formation,  its  direct  action  is  not  of  great 
importance  and  is  adequately  disposed  of  in  paragraph  27. 


18  NATURE  AND  PROPERTIES  OF  SOILS 

armed.  Its  cutting  power,  therefore,  depends  on  the  amount 
of  sediment  that  it  carries  and  on  its  velocity  of  flow. 
Erosion  by  water  deserves  particular  attention,  as  its  denud- 
ing effects  are  very  rapid  when  geologically  viewed.  Most 
of  the  changes  in  topography  are  due  to  such  activity.  The 
material  swept  away  is  partly  in  suspension  and  partly  in 
solution.1  The  Appalachian  Mountains,  whose  uplift  was 
complete  in  Carboniferous  times,  have  lost  vastly  more  of  their 
mass  than  now  remains  in  view. 

While  most  of  the  debris  from  the  ancient  erosive  cycles 
has  been  changed  to  rock  or  has  become  a  noticeable  charac- 
teristic of  ocean  water,  remnants  persist.  To  these  remnants 
rivers,  lakes  and  oceans  are  making,  year  by  year,  substantial 
additions.  The  cutting,  carrying  and  depositing  activity  of 
streams  produce  alluvial  soils  of  which  the  Mississippi  flood 
plain  is  a  well  known  example.  Deltas  built  into  oceans,  lakes 
and  gulfs  represent  stream  activity  under  different  condi- 
tions, while  uplifted  continental  shelves  are  often  bedded  with 
erosive  products.  The  delta  and  marine  soils  of  the  Atlantic 
and  Gulf  coastal  plains  afford  examples  of  the  latter  types 
of  soil  production.  Even  the  pounding,  grinding  and  sorting 
activities  of  waves  in  ocean  and  lake  are  no  mean  factors  in 
the  mechanics  of  soil  formation. 

12.  Glacial  action. — Ice  at  the  present  time,  especially 
in  temperate  regions,  is  of  little  importance  in  soil  forma- 
tion. Nevertheless,  at  a  comparatively  recent  date  geolog- 
ically, it  had  much  to  do  with  the  preparation  and  deposition 
of  soil  materials  over  great  areas  in  central  and  northern 
North  America,  northern  Europe  and  the  British  Isles.  Dur- 
ing the  Great  Ice  Age  immense  continental  glaciers  succes- 
sively invaded  these  regions,  much  as  the  ice  cap  is  over- 

1  The  chemical  denudation  by  streams  is  generally  spoken  of  as  corro- 
sion. Abrasion  is  applied  to  the  wear  of  the  stream  load  upon  its 
channel  and  of  the  particules  in  suspension  upon  themselves.  Erosion 
is  a  broader  term  including  corrosion  and  abrasion  as  well  as  trans- 
portation. 


SOIL-FORMING  PROCESSES  19 

riding  Greenland  to-day.  Of  great  thickness  and  weight  and 
impelled  southward  by  tremendous  pressure,  these  ice  sheets 
swept  away  the  old  soil  mantle  and  ground  the  underlying 
rocks  with  irresistible  energy.  The  heterogeneous  debris,  im- 
bedded in  the  ice,  only  served  to  enhance  the  cutting  power 
of  the  slowly  moving  mass.  Hundreds  of  square  miles  were 
covered  and  as  the  ice  was  often  several  thousand  feet  thick, 
mountains  as  well  as  hills  were  over-ridden.     (See  Fig.  3.) 

In  the  melting  back  of  these  tremendous  ice  sheets,  the 
accumulated  debris  was  of  necessity  left  behind.  When  the 
ice  retreat  was  rapid,  the  deposit  was  comparatively  thin  and 
uniform.  When  a  halt  occurred,  the  material  was  left  in 
irregular  hummocks.  It  is  hardly  necessary  to  state  that  the 
soil  developed  from  the  former  deposit  is  the  more  important 
agriculturally,  due  to  its  level  topography  and  wide  extent. 
The  area  of  the  latter  is  fortunately  small.  The  streams 
flowing  from  the  ice  fronts  were  no  insignificant  feature  of 
the  glacial  phenomena.  Such  streams  were  heavily  laden  with 
sediment,  which  was  distributed  far  and  wide  in  regions  miles 
beyond  the  ice  front. 

In  whatever  manner  the  glacial  debris  was  laid  down  it  is 
necessary  to  note  that  such  deposits  were  soil  material,  not 
soil.  Chemical  action  in  all  its  complexity  and  the  interven- 
tion of  plants  and  animals,  especially  the  former,  were  neces- 
sary before  a  true  soil  could  be  born,  a  soil  still  in  its  youth 
and  covering  in  the  United  States  alone  over  500,000  square 
miles.     (See  Fig.  3,  page  20.) 

13.  The  influence  of  wind. — Wind,  like  water  and  ice, 
has  both  cutting  and  carrying  power.  The  fluting  of  rocks, 
the  polishing  of  stones,  and  the  undermining  of  cliffs  are  of 
such  frequent  note  as  to  require  but  brief  mention.  There 
seems  no  escape  from  the  conclusion  that  wind  is  engaged  in 
rock  disintegration.  Its  geological  function  in  arid  regions 
seems  similar  to  that  of  running  water  in  humid  lands. 

It  is,  however,  as  a  transporting  agency  of  fine  materials 


20 


NATURE  AND  PROPERTIES  OF  SOILS 


Fig.  3. — Sketch  map  of  North  America  showing  the  approximate  south- 
ward extension  of  the  great  ice  sheets  and  the  three  centers  of 
accumulation. 


SOIL-FORMING  PROCESSES  2i 

that  the  wind  is  of  especial  importance  in  soil  formation.  The 
movement  of  sand  and  dust  in  both  humid  and  arid  regions 
is  almost  incessant.  In  desert  storms  200  tons  of  materials 
have  been  known  to  float  over  every  acre  of  land.  The  finer 
particles  travel  for  miles  in  a  very  short  time.  Southern  Italy 
has  received  as  much  as  one  inch  of  dust  from  Africa  during 
a  single  storm.  The  movement  of  sand  dunes  is  but  another 
evidence  of  the  transporting  power  of  air  in  motion. 

Wind  as  an  agency  in  soil  formation  would  perhaps  receive 
much  less  attention  were  it  not  for  the  existence  of  large 
areas  of  a  certain  silty  soil  called  loess.  This  soil  exists  along 
the  Rhine  both  in  France  and  Germany,  in  southern  Russia, 
in  Roumania,  in  China  and  in  central  United  States.  This 
material,  as  well  as  the  adobe  of  our  arid  Southwest  is  con- 
sidered as  largely  wind  laid.  Since  the  loess  is  highly  fertile 
and  of  great  agricultural  importance,  added  attention  is  thus 
directed  towards  wind  as  a  soil-forming  agency.    (See  Fig.  4.) 

14.  Change  in  temperature. — Variations  of  temperature, 
especially  if  sudden  or  wide,  greatly  augment  the  denuding 
actions  of  water,  ice,  and  wind.  Rocks  and  soil  become  heated 
during  the  day  and  at  night  often  cool  much  below  the  tem- 
perature of  the  air.  This  warming  and  cooling  is  particularly 
effective  as  a  disintegrating  agent.  Rocks  are  mineral  aggre-i 
gates,  the  minerals  varying  in  their  coefficients  of  expansion.) 
With  every  temperature  change  differential  stresses  are  set 
up,  which  eventually  must  produce  cracks  and  rifts,  since  the 
minerals  never  assume  their  original  position.  Incipient  focii 
for  further  physical  and  chemical  change  are  thus  established. 
Although  the  expansion  coefficient  of  rock  is  low,  it  must  be 
remembered  that  very  large  surfaces  are  involved.  Moreover, 
it  is  the  multiplicity  of  the  rifts  rather  than  their  magnitude 
that  is  important. 

The  influence  of  temperature  change  is  manifested  on  rocks 
in  another  way.  Due  to  slow  conduction  the  outer  surface 
of  a  rock  often  maintains  a  markedly  different  temperature 


22 


NATURE  AND  PROPERTIES  OF  SOILS 


than  the  inner  and  more  protected  portions.  This  differential 
heating  tends  to  set  up  lateral  stresses  which  may  cause  the 
surface  layers  to  peel  away  from  the  parent  mass.  This  phe- 
nomena is  spoken  of  as  exfoliation.    The  differential  expansion 


Fig.  4. — Approximate  distribution  of  loess  in  central  United  States. 


of  the  rock  minerals  of  course  plays  a  part  in  this  disintegra- 
tion, although  exfoliation  readily  occurs  in  rocks  which  are 
more  or  less  homogeneous.  While  this  form  of  weathering 
may  go  on  alone,  it  is  much  accelerated  by  chemical  action 
and  the  prying  of  freezing  water. 


SOIL-FORMING  PROCESSES  23 

One  peculiarity  of  pure  water  is  that  its  maximum  density 
occurs  at  39.2  deg.  F.  From  this  point  the  volume  increases 
as  the  temperature  is  lowered.  Ice,  which  forms  at  32  deg.  F., 
thus  occupies  a  greater  space  than  the  water  from  which  it 
was  derived.  The  force  developed  by  freezing  is  equivalent 
to  about  150  tons  to  the  square  foot  or  a  pressure  of  141 
atmospheres.  The  cracks  and  crevices  of  surface  rocks  in 
humid  regions  are  from  time  to  time  filled  with  moisture. 
Rocks  below  the  surface  contain  water  continuously.  The 
change  of  this  water  from  a  liquid  to  a  solid  always  produces 
marked  disintegration.  Mountain-top  rubble,  talus  slopes, 
alluvial  fans,  and  similar  formations  are  evidences  of  such 
action.  The  load  of  sediment  carried  by  streams  is  often  due 
to  the  prying  action  of  temperature  change,  especially  where 
crevice  water  is  present. 

This  action  of  temperature  is  by  no  means  ended  when  a 
soil  is  produced.  Freezing  and  thawing  is  of  tremendous  im- 
portance in  bettering  the  physical  condition,  especially  of 
heavy  soils.  It  is  to  such  forces  that  the  farmer  owes  the 
good  tilth  of  his  land.  In  addition  it  must  be  noted  that  the 
rapidity  of  chemical  change  is  largely  a  function  of  temper- 
ature. The  concentration  of  the  soil  solution  and  the  avail- 
ability of  the  nutrient  elements  thus  come  under  the  influence 
of  this  apparently  simple  force. 

15.  Plants  and  animals. — While  plants  and  animals  unite 
their  activities  with  the  processes  already  mentioned,  their 
influence  is  confined  largely  to  the  soil  and  the  soil  material. 
Simple  plants  such  as  mosses  and  lichens  grow  upon  exposed 
rock,  there  to  catch  dust  and  dirt  until  a  thin  film  of  highly 
organic  material  accumulates.  Higher  plants  sometimes  exert 
a  prying  effect  on  rock,  which  results  in  some  distintegration. 
Such  influences,  however,  are  of  but  little  import  in  soil  for- 
mation compared  to  the  drastic  activities  of  water,  wind,  ice 
and  temperature  change. 

In  the  soil,  roots  by  their  ramifications  promote  aeration 


24  NATURE  AND  PROPERTIES  OF  SOILS 

and  drainage,  as  well  as  an  accumulation  and  distribution  of 
organic  materials.  Lichens,  mosses,  and  algae  play  their  parts 
in  a  similar  manner.  It  must  be  noted,  however,  that  while 
plants  tend  to  preserve  and  improve  the  soil  tilth,  their  action 
in  this  respect  is  not  wholly  physical.  Decay  due  largely  to 
bacterial  action  is  necessary  before  the  accumulated  organic 
matter  can  improve  to  any  marked  degree  the  physical  con- 
dition of  the  soil.  This  is  only  one  of  the  many  examples 
illustrating  the  cooperation  of  physical  and  chemical  changes 
incident  to  soil  formation. 

Animals  influence  the  soil  physically  by  their  burrowing 
propensities.  Gophers,  squirrels,  ants,  and  the  like  mix  and 
open  up  the  soil,  thus  providing  for  the  circulation  both  of 
air  and  water.  Other  soil  forces,  both  physical  and  chemical, 
are  markedly  encouraged  thereby.  Earth  worms  produce 
similar  effects.  They  not  only  pass  great  quantities  of  soil 
through  their  bodies,  but  they  carry  much  to  the  surface. 
This  has  been  estimated  as  amounting  to  one  or  two  surface 
inches  in  a  decade.  Man  also  is  producing  important  physical 
changes  on  the  soil  and  soil  material.  The  plowing  under  of 
green-manures,  crop  residues  and  farm  manure,  the  addition 
of  lime  and  fertilizers  and  the  tillage  incident  to  cropping 
have  much  to  do  with  the  physical  changes,  which  are  con- 
tinually occurring  in  the  soil. 

16.  Oxidation  and  deoxidation. — Scarcely  has  the  disin- 
tegration of  rock  begun  than  its  decomposition  is  also  appar- 
ent. This  is  especially  noticeable  in  humid  regions  where  the 
chemical  and  physical  processes  of  soil  formation  are  par- 
ticularly active  and  markedly  accelerate  each  other.  Of  the 
chemical  forces,  oxidation  is  usually,  especially  near  the  sur- 
face, the  first  to  be  noticed.  It  is  particularly  manifest  in 
rocks  carrying  iron  in  the  sulfide,  carbonate  or  silicate  forms. 
The  sulfide,  although  widespread,  is  less  important  in  pro- 
moting rock  decay  than  the  other  combinations.  The  oxida- 
tion of  iron  in  any  form  is  indicated  by  a  discoloration  of  the 


SOIL-FORMING  PROCESSES  25 


affected  rock,  which  from  the  first  is  streaked  with  iron  oxide. 
The  mica,  amphibole,  pyroxene  and  garnet  groups  are  par- 
ticularly affected,  until,  as  the  process  continues,  these  min- 
erals waste  away  into  unrecognizable  forms  so  weakening  the 
rock  as  to  cause  it  to  crumble  easily.  The  way  is  now  open 
for  vigorous  chemical  and  physical  changes  of  all  kinds.  Oxi- 
dation may  be  illustrated  chemically,  using  olivine  as  the 
mineral  decomposed.  It  is  to  be  noted  that  the  first  step  is 
the  assumption  of  water  and  the  production  of  serpentine  and 
ferrous  oxide.    The  latter  quickly  changes  to  the  susquioxide. 

3MgFeSi04+2H20=H4MggSi209+Si02+3FeO 
Olivine     Water     Serpentine     Silica    Ferrous 

Oxide 
4FeO  +  02  =  2Fe203  (red) 
Ferrous     Oxygen  Ferric  Oxide 
Oxide 

Deoxidation  is  the  reverse  of  oxidation,  being  a  reduction 
of  the  amount  of  oxygen  present  in  the  compound.  With 
hematite  it  might  occur  as  follows : 

2Fe203  —  02  =  4FeO 
Ferric  Oxide  Oxygen  Ferrous  Oxide 

In  a  similar  way,  other  oxides  and  salts  may  be  reduced  by 
the  withdrawal  of  oxygen.  This  action  occurs  in  poorly 
drained  soils  or  in  soil  very  rich  in  organic  matter.  It  is 
generally  apparent  in  forest  soils  just  below  the  organic  sur- 
face layer.  Here  the  leaching  downward  of  small  quantities 
of  organic  acids  has  been  sufficient  to  develop  a  definite  grey- 
ish zone,  varying  both  in  color  and  depth.  The  bleaching  of 
sands,  shales,  sandstones,  and  clays  may  often  be  due  to 
deoxidation  rather  than  the  actual  removal  of  ferric  iron. 
No  great  importance  need  be  attached  to  deoxidation  either 
in  soil  formation  or  in  the  chemical  processes  which  continue 
to  affect  the  soil  after  it  is  definitely  developed. 


26  NATURE  AND  PROPERTIES  OF  SOILS 

17.  Carbonation. — The  process  of  oxidation  is  almost  al- 
ways accompanied  by  the  action  of  carbon  dioxide.  This  gas 
is  a  constituent  of  the  air  and  is  a  product  of  the  organic 
decay  which  vigorously  progresses  in  most  soils.  It  occurs 
in  large  amounts  in  rain  water,  especially  in  warm  climates. 
It  increases  the  solvent  action  of  water  by  actively  engaging 
in  chemical  reactions,  producing  carbonates  and  bicarbonates 
with  the  various  rock  and  soil  bases.  The  decomposition  of 
orthoclase  and  muscovite  mica  into  kaolinite  and  carbonates 
is  as  follows: 

2KAlSi308  +  2H20  +  C02  =  H4Al2Si209  +  K2C03  +  4Si02 
Orthoclase       Water     Carbon      Kaolinite     Potassium     Silica 
Dioxide  Carbonate 

2H2KAl3Si3012  +  C02  +  4H20  ±  3H4Al?Si209  +  K2COs 

Muscovite  Carbon      Water         Kaolinite      Potassium 

Dioxide  Carbonate 

Under  certain  conditions  decarbonation  may  occur.  When- 
ever the  processes  of  weathering  produce  either  mineral  or 
organic  acids  carbonates  are  rapidly  decomposed.  The 
presence  of  unsaturated  aluminum  silicates  may  also  rapidly 
promote  decarbonation  by  absorbing  the  base  and  liberating 
the  acid  radical.  This  latter  reaction  is  of  especial  importance 
in  soil. 

18.  Hydration. — All  the  chemical  transformations  above 
discussed  depend  on  the  presence  of  a  certain  amount  of 
water,  especially  if  rapid  changes  are  to  occur.  The  illus- 
trative reactions  already  cited  indicate  this.  Oxidation  pro- 
ceeds but  slowly  in  a  dry  atmosphere,  water  being  necessary 
as  a  catalytic  agent.  In  the  carbonation  of  the  potash  of 
orthoclase  and  mica,  water*  enters  into  the  reactions,  produc- 
ing not  only  kaolinite  but  also  potassium  hydroxide,  which  is 
later  changed  to  the  carbonate. 

Water  functions  in  the  chemical  changes  of  rock  and  soil 


SOIL-FORMING  PROCESSES  27 

in  another  way — as  water  of  combination.1  The  process  is 
called  hydration.  While  hydration  usually  proceeds  or  ac- 
companies oxidation  and  carbonation,  thus  making  them  pos- 
sible, it  often,  unlike  these  transformations,  occurs  at  great 
depths  and  may  be  practically  the  only  change  that  the  rock 
minerals  have  undergone.  Many  minerals,  especially  the  oliv- 
ine, feldspar  and  mica  groups,  are  so  affected.  They  become 
soft  and  lose  their  luster  and  elasticity  on  the  assumption  of 
this  chemically  combined  water.  Considerable  increase  of 
bulk  occurs  during  the  transition  of  the  rock  to  soil.  The 
latter  change  has  no  small  physical  significance.  This  hydra- 
tion is  particularly  effective  in  encouraging  other  kinds  of 
chemical  decay.  In  addition  to  the  examples  already  cited, 
the  change  of  hematite  to  limonite,  which  occurs  to  a  greater 
or  less  degree  in  every  soil  where  the  sesquioxide  is  present, 
is  worthy  of  note: 

2Fe203  +  3H20  =  2Fe203  .  3H20 
Hematite      Water    Limonite  (yellow) 

When  the  products  of  weathering  dry  out  due  to  varying 
weather  conditions,  dehydration  may  occur.  Thus  limonite 
may  readily  reduce  to  a  lower  hydrate  or  to  hematite. 

19.  Solution. — It  is  quite  evident  that  while  weathering 
and  erosion  produce  many  compounds  of  a  very  complex  char- 
acter, there  is  a  tendency  toward  simplification  and,  as  water 
is  universally  present,  some  solution  occurs.  Such  bases  as 
calcium,  magnesium,  sodium  and  potassium  are  found  in  the 
water  that  circulates  in  rocks,  soil  materials  and  soils.  These 
bases,  when  in  solution,  are  generally  combined  as  chlorides, 
phosphates,  nitrates,  carbonates,  and  the  like.  Carbon  dioxide 
intensifies  to  a  marked  degree  the  solvent  action  of  water  and 
consequently  increases  its  power  as  a  weathering  agent.    The 

1  Note  carefully  the  difference  between  hydration  and  the  production 
of  an  hydroxide.     The  former  is  the  more  important  as  a  soil  phenome- 


28  NATURE  AND  PROPERTIES  OF  SOILS 

atmosphere  carries  about  .03  per  cent,  of  carbon  dioxide  by 
volume,  while  considerable  amounts  are  brought  down  on 
rocks  and  soil  by  rain  and  snow.  Traces  of  nitric  and  sul- 
furic acid  are  also  found  in  rain  water.  The  carbon  dioxide 
produced  within  the  soil  by  decaying  organic  matter  keeps 
the  concentration  of  this  gas  high  at  points  where  it  can  act 
most  effectively. 

Solution,1  accelerated  both  by  mechanical  and  chemical 
means,  is  of  particular  importance  in  two  directions.  In  the 
first  place,  it  allows  a  continual  loss  of  plant  nutrients  not 
only  as  the  soil  is  being  formed  but  after  it  becomes  a  proper 
medium  for  plants.  This  constant  drain  accounts  for  the 
deficiency  of  certain  elements  in  the  soil  and  the  need  in  cer- 
tain cases  of  such  additions  as  lime  and  fertilizers.  On  the 
other  hand,  this  solution,  however  wasteful,  is  necessary  since 
plants  absorb  nutrients  from  the  soil  only  in  soluble  form. 
The  concentration  and  composition  of  the  materials  in  the 
soil  water  is  thus  a  function  of  solution,  which  is  a  culmina- 
tion of  the  activities  of  the  soil  processes  already  discussed. 

20.  General  statement  of  soil  formation. — By  a  very 
complicated  coordination  the  mechanical  and  chemical  forces 
of  weathering  reduce  the  solid  rock  to  small  fragments  and 
mix  therein  the  necessary  organic  matter.  The  process  slowly 
proceeds  until  a  suitable  medium  for  the  growth  of  higher 
plants  is  produced.  As  a  rule,  the  chemical  processes  are  in- 
complete and  all  stages  of  decay  are  exhibited.  This  is  for- 
tunate, as  solution  may  thereby  continue  to  renew  the  nutrients 
in  the  soil-water  for  a  long  period  and  thus  maintain  the 
continuous  productivity  of  the  soil. 

The  products  of  disintegration  and  decomposition  are  com- 
monly classified  into  two  general  groups,  sedentary  and  trans- 

1  While  the  formula  for  water  is  generally  given  as  H,0  the  molecule 
is  not  as  simple  as  this,  being  at  low  temperature  as  high  as  (H20)4. 
The  remarkable  power  of  water  as  a  solvent  may  be  due  to  extra  oxygen 
valences  as  well  as  to  the  high  dielectric  constant  which  favors  ioniza- 
tion, thus  hastening  chemical  reaction. 


SOIL-FORMING  PROCESSES 


29 


ported.  The  former  remains  in  place,  being  the  rock  residuum 
in  which  organic  matter  accumulates.  Residual  clay  is  an 
example.  The  second  group,  on  the  other  hand,  in  addition 
suffers  transportation  and  is  represented  by  the  soils  arising 
from  glacial  drift,  alluvial  accumulations,  aeolian  deposits, 
and  the  like.  In  the  first  case,  the  soil  is  derived  from  a  single 
lithologic  unit;  in  the  second  place,  the  assorted  and  blended 
materials  are  from  many  sources.  A  general  statement  of 
the  formation  of  a  residual  soil1  is  obviously  the  easier  to 


Fig.  5. — The  gradual  transition  of  country  rock  into  residual  soil  by 
weathering  in  situ. 

make.  Such  a  statement  adequately  covers  every  process  in 
the  production  of  a  transported  soil  except  the  disintegra- 
tion, assortment,  and  solution  due  to  translocation.  (See 
Fig.  5.) 

'  *  The  changes  that  a  rock  undergoes  in  forming  a  residual 
soil  are  first  a  physical  breaking  down,  accompanied  by  certain 
chemical  transformations,  which  consist  in  the  hydration  of 
a  portion  of  the  feldspars,  micas  and  similar  minerals;  the 

1Buckman,  H.  O.,  The  Formation  of  Besidual  Clay;  Trans.  Amer. 
Cer.  Soc,  Vol.  XIII,  p.  362.     Feb.,  1911. 


30  NATURE  AND  PROPERTIES  OF  SOILS 

oxidation  and  hydration  of  a  part  of  the  combined  iron ;  and 
a  carbonation  and  solution  of  a  large  proportion  of  the  soluble 
bases.  These  processes  are  hastened  and  the  whole  mass 
evolved  into  a  soil  by  the  admixture  and  decay  of  certain 
amounts  of  organic  matter."1 

21.  Variation  of  soil  formation  with  climate. — It  may 
be  seen  readily  that  the  activity  of  the  various  soil-forming 
agencies  will  fluctuate  with  climate.  A  comparison  of  weath- 
ering and  erosion  in  an  arid  and  a  humid  region  will  illustrate 
the  point  at  issue.  Under  arid  conditions,  the  physical  forces 
will  dominate  and  the  resultant  soil  will  be  coarse.  Tempera- 
ture changes,  wind  action  and  the  influence  of  animals  will 
be  almost  the  sole  agents.  In  a  humid  region,  however,  the 
forces  are  more  varied  and  practically  the  full  quota  will  be 
at  work.  Chemical  decay  will  accompany  disintegration  and 
the  result  will  be  shown  in  the  greater  fineness  of  the  product. 
The  separate  minerals  will  also  show  the  change  of  color  and 
loss  of  luster  so  characteristic  of  chemical  action.  A  granite, 
for  example,  is  a  very  insoluble  rock,  compared  with  a  lime- 
stone, and  in  a  humid  region,  where  chemical  agencies  are 
dominant,  it  will  be  markedly  more  resistant.  If,  however, 
these  rocks  are  exposed  in  an  arid  region,  where  physical 
weathering  is  potent,  the  results  will  be  entirely  different. 
The  limestone,  being  homogeneous,  will  not  be  affected  mark- 
edly by  temperature  changes,  but  the  stresses  set  up  in  granite 
must  ultimately  reduce  it  to  fragments. 

Arid  soils,  besides  being  rather  coarse,  are  generally  rather 
uniform,  there  being  little  difference  between  soil  and  subsoil. 
The  soils  of  humid  regions  are  usually  of  fine  texture,  par- 
ticularly in  residual  sections,  since  the  chemical  agencies  have 

1It  is  well  to  remember  that  synthetic  processes  as  well  as  forces  of 
simplification  and  dissolution  are  active  in  soil  formation.  The  soil 
features  that  result  are  of  two  kinds,  hereditary  and  acquired.  The 
former  develop  through  geological  forces,  the  latter  through  the  activity 
of  true  soil  processes. 


SOIL-FORMING  PROCESSES 


31 


been  so  active.  Various  colors  may  develop  because  of  oxida- 
tion, hydration,  and  the  presence  of  organic  matter.  Such 
soils  usually  are  not  excessively  deep,  and  are  likely  to  be 
underlaid  by  subsoils  heavier  than  the  surface.  The  general 
physical  condition  and  tilth  of  arid  soil  is  uniformly  better 
than  that  of  regions  of  plentiful  rainfall. 

Chemically,  because  of  less  leaching,  the  arid  soils  contain 
more  of  the  important  mineral  elements.  The  following 
analyses  bring  out  the  differences  in  a  striking  manner : 


Table  II 

COMPARATIVE    ANALYSES    OF    ARID    AND    HUMID    SOILS1 


Arid  Soils 

Humid  Soils 

Constituents 

Average  of 

Average  of 

313  Samples 

466  Samples 

Insoluble 

77.82 

88.24 

A1203 

7.89 

4.30 

Fe203 

5.75 

3.13 

CaO 

1.36 

.11 

K20 

.73 

.22 

PA 

.12 

.11 

MgO 

1.41 

.23 

Volatile 

4.94 

3.64 

It  is  immediately  apparent  that  the  arid  soil  is  poorer  in 
silica  than  the  humid  soil,  but  richer  in  iron  and  alumina,  in- 
dicating a  less  weathered  condition  of  the  feldspars.  Due  to 
a  greater  amount  of  leaching,  the  humid  soil  is  much  lower 
in  phosphoric  acid,  lime,  magnesia,  and  potash.  The  humus 
in  arid  soils  is  somewhat  lower  than  in  the  soils  under  better 


1Hilgard,  E.  W.,  Die  Boden  arider  und  humider  Lander;  Internat. 
Mitt.  Bodenkunde,  Bd.  I,  pp.  415-529.     1912. 


32  NATURE  AND  PROPERTIES  OF  SOILS 

conditions  of  rainfall,  as  one  would  naturally  expect.  The 
amount  of  easily  soluble  material  is  higher  in  arid  regions, 
due  to  the  lack  of  rain  and  the  tendency  for  soluble  salts  to 
accumulate.  Biologically,  organisms  are  active  at  greater 
depths1  in  arid  than  in  humid  regions,  because  of  the  loose 
structure  of  arid  soils  and  because  of  their  good  aeration. 
Such  soils  are  seldom  water-logged  except  from  improper  ir- 
rigation. In  humid  regions  bacterial  action  is  limited  very 
largely  to  the  surface  foot  of  soil,  since  only  there  are  the 
aeration  and  the  food  conditions  adequate.  The  intensity  of 
biological  activity  in  arid  soils  is  very  largely  governed  by 
moisture,  and  when  moisture  conditions  are  satisfied,  bacterial 
changes  may  be  expected  to  take  place  rapidly. 

22.  Special  cases  of  soil  formation. — Having  compared 
the  weathering  of  granite  and  limestone  under  different  cli- 
matic conditions,  it  is  interesting  to  note  the  quantitative  chem- 
ical changes  of  these  rocks  as  they  are  reduced  residually  to 
soil  under  humid  conditions.  The  following  analyses2  indicate 
the  elements  that  are  likely  to  be  lost  to  the  greatest  extent 
during  the  process.     (See  Tables  III  and  IV,  page  33.) 

The  soil  resulting  from  the  decay  of  the  granite  was  a  deep 
red  clay,  with  numerous  quartz  grains  present.  The  soil 
from  the  limestone  was  very  plastic  and  high  in  silicate  silica. 
Leaching  has  probably  gone  on  to  a  very  great  extent  in  both 
soils.  It  is  noticeable  in  both  cases  that  the  bases,  such  as 
calcium,  magnesium,  sodium,  and  potassium,  have  suffered 
severe  losses.  The  carbonate  has  almost  wholly  disappeared 
from  the  limestone  clay,  indicating  that  a  residual  soil  from 
such  a  rock  will  probably  need  an  application  of  lime.  (See 
Figs  6  and  7,  pages  34  and  35.) 

1Lipman,  C.  B.,  The  Distribution  and  Activities  of  Bacteria  m  Softs 
of  the  Arid  Region;  Univ.  Calif.,  Pub.  in  Agr.  Sci.,  Vol.  I,  No.  1,  pp. 
1-20.     1912. 

2  Merrill,  G.  P.,  Weathering  of  Micaceous  Gneiss;  Bui.  Geol.  Soe. 
Amer.,  Vol.  8,  p.  160.     1879. 


SOIL-FORMING  PROCESSES 


33 


Table  III 

FRESH  GRANITE  AND   ITS  RESIDUAL  CLAY 


Constituents 

Eock 

Soil 

Percentage 
Lost1 

Si02 

60.69 

45.31 

52.45 

A1203 

16.89 

26.55 

.00 

Fe203 

9.06 

12.18 

14.35 

CaO 

4.44 

.00 

100.00 

MgO 

1.06 

.40 

74.70 

K20 

4.25 

1.10 

83.52 

Na20 

2.82 

.22 

95.03 

P205 

.25 

.47 

.00 

Ignition 

.62 

13.75 

gam 

Table  IV 

VIRGINIA   LIMESTONE    AND   ITS   RESIDUAL    CLAY2 


Constituents 

Rock 

Soil 

Percentage  * 
Lost 

Si02 

7.41 

57.57 

27.30 

A1203 

1.91 

20.44 

.00 

Fe203 

.98 

7.93 

24.89 

CaO 

28.29 

.51 

99.83 

MgO 

18.17 

1.21 

99.38 

K20 

1.08 

4.91 

57.49 

Na20 

.09 

.23 

76.04 

PA 

.03 

.10 

68.78 

co2 

41.57 

.38 

99.15 

H20 

.57 

6.69 

gain 

*The  percentage  loss  of  any  constituent  is  calculated  as  follows: 
Ax  100 

=  X        100  —  X  =  %  Lost. 

C 

BX- 
D 

A  =  %  any  constituent  in  residual  material. 
B  ==  %  same  constituent  in  fresh  rock. 
C  =  %  of  the  constant  constituent  in  residual  soil. 
D  =  %  of  the  constant  constituent  in  fresh  rock. 
aDiller,  J.  S.,  Educational  Series  of  Bock  Specimens;   U.  S.   Geol. 
Survey,  Bui.  150,  p.  385.     1898. 


34  NATURE  AND  PROPERTIES  OF  SOILS 

The  analyses  indicate  that  the  soil  from  the  granite  does 
not  differ  greatly  from  the  original  rock,  except  in  the  loss  of 
bases,  assumption  of  water,  and  increase  of  organic  matter. 
The  soil  from  the  limestone  presents  greater  differences,  due 


\::::_: 


M'°    1.41 


K20 
Na20 


I  1.1 1 
J2.8C 
I    .21 

r 

U5.1 


1  I GRANITE 

■■■RESIDUAL  SOIL 


6D 
H20 

"15* 


FlG.   6.  —  Diagram   showing   the   composition   of   fresh   granite   and  its 
residual  soil.     Note  the  marked  hydration  of  the  soil. 


to  the  disappearance  of  the  calcium  carbonate.  The  analyses 
of  the  two  soils  resemble  each  other  rather  closely  in  spite  of 
their  widely  different  sources.  Since  weathering,  especially 
residual   weathering,   causes  a  loss  of  basic  materials  and 


SOIL-FORMING  PROCESSES  35 

thereby  favors  the  accumulation  of  silica,  alumina  and  iron, 
all  soils  as  they  age  tend  to  approach  each  other  in  chemical 
composition.  Yet,  owing  to  a  difference  in  the  adjustment 
of  the  forces  at  work  and  to  the  time  element,  no  two  soils 


% 


?•»  fcSf 


f^Oj+j  2.91=3 

A1Z03  \28AWm 


CaO-W46.0C 
MgO    I    1.71 


KzO+(    1.0  D 
1    5-1" 


Na20 


«*  r " 


2  LIMESTONE 
I  RESIDUAL  SQ>IL 


Fig.  7. — Diagram  showing  the  composition  of  limestone  and  its  residual 
soil.  Note  the  excessive  loss  of  lime  and  carbon  dioxide  in  soil 
formation. 


will  ever  be  exactly  alike.  Soils  will  differ  from  the  original 
rock  and  from  one  another  according  to  the  intensity  and 
character  of  the  weathering  and  erosive  forces  and  to  the 
constitution  of  the  parent  minerals. 


36  NATURE  AND  PROPERTIES  OF  SOILS 

23.  Red  and  yellow  colors  of  soil.1 — The  presence  of 
iron,  as  already  noted,  is  a  very  important  factor  in  rock 
weathering,  and  the  discoloration  due  to  its  presence  is  an 
unfailing  indication  of  chemical  decay.  The  iron  in  minerals 
occurs  usually  as  ferrous  oxide,  which  is  soluble,  especially 
if  the  water  circulating  among  the  rock  fragments  carries 
carbon  dioxide.  When  this  water  comes  in  contact  with  the 
air  its  excess  of  carbon  dioxide  is  discharged  and  the  oxides 
and  carbonates  of  iron  are  deposited.  Under  this  condition 
oxidation  goes  on  rapidly,  and  the  iron  passes  to  the  ferric 
state  and  becomes  insoluble.  Thus  it  may  be  seen  that  iron 
imparts  a  fatal  weakness  to  rocks  and  minerals  in  which  it 
exists,  due  to  its  solubility;  yet  from  the  oxidation  that  it 
undergoes  it  tends  to  persist  and  accumulate  in  soils.  The 
more  iron  a  mineral  or  rock  contains  the  more  susceptible  it 
is  to  weathering. 

The  red  and  yellow  soils  of  the  southern  states  frequently 
excite  comment,  especially  as  a  difference  in  fertility  is  popu- 
larly recognized,  the  red  surface  soil  with  a  red  subsoil  being 
considered  more  fertile  than  a  similar  soil  with  a  yellow  sub- 
soil. This  is  probably  due  to  differences  in  hydration  of 
the  iron  oxides.2 

The  soil  temperatures,  particularly  in  tropical  and  sub- 
tropical regions,  have  first  tended  fully  to  oxidize  and  hydrate 
the  iron,  and  then  to  dehydrate  the  soil  at  the  surface  into 
the  deep  red  color,  leaving  the  subsoil  yellow  and  causing 
the  contrasts  so  markedly  evident.  Soils  having  a  yellow 
surface  soil  are  generally  considered  to  be  older  and  more 
weathered  than  those  where  the  red  is  well  developed.    When 

1Eobinson,  W.  O.,  and  McCaughey,  W.  J.,  The  Color  of  Soils;  U.  S. 
Dept.  Agr.,  Bur.  Soils,  Bui.  79,  p.  21.     1911. 

2  Crosby,  W.  O.,  Colors  of  Soils;  Proc.  Boston  Soc.  Nat.  Hist.,  Vol.  23, 
pp.  219-222.  1875.  Merrill,  G.  P.,  Boclcs,  Rock  Weathering  and  Soils; 
p.  375.  New  York,  1906.  Van  Bemmelen,  J.  M.,  Beitrage  zur  Eenntnis 
der  VerwitterungsproduTcte  der  Silicate  in  Ton-,  VulJcanischen-,  und 
Laterite-Boden;  Zeit.  Anorg.  Chem.,  Bd.  42,  Steite  290-298.     1904. 


SOIL-FORMING  PROCESSES 


37 


these  old  residual  soils  are  poorly  drained  a  well  denned 
mottling  develops,  especially  in  the  subsoil,  due  to  the  ir- 
regularities of  aeration. 

The  compositions  of  hematite  and  of  the  limonite  group 
indicate  the  possibility  of  a  progressive  change  from  red  to 
yellow  by  hydration : 

Hematite     Fe203  Red 

'Turgite    2Fe203.   B20 

Limonite  Goethite    Fe203 .   H20 

Group i  Limonite    2Fe203.3H20 

Xanthosiderite  .  Fe203.2H20 
.Limnite    Fe203.3H20     Yellow 

24.  Practical  relationships  of  weathering. — Soil-form- 
ing processes  fortunately  remain  intensely  active  after  the 
soil  has  been  produced.  The  physical  agencies  especially 
tend  to  loosen  and  fine  the  soil,  contributing  largely  to  its 
tilth.  The  farmer  encourages  such  influences  by  plowing  his 
land  and  by  other  tillage  operations.  The  addition  of  organic 
matter  is  another  means  whereby  these  physical  changes  may 
be  influenced.  Granulation  in  a  clay  soil  is  due  almost  en- 
tirely to  natural  agencies.  Were  it  not  for  such  activities 
the  soil  would  soon  become  physically  unfit  as  a  foothold  for 
plants.  The  continual  chemical  changes,  culminating  in  solu- 
tion, provide  the  soil-water  with  plant  nutrients  not  only 
in  suitable  concentration  but  in  correct  proportion.  By 
slow  processes,  over  geologic  periods,  Nature  has  provided 
us  with  soil  and  by  the  same  slow  processes  Nature  is  at- 
tempting to  maintain  the  fertility  of  her  creation.  The  en- 
couragement and  control  of  such  agencies  is  of  no  small 
moment  in  practical  soil  management. 


CHAPTER  III 

THE  GEOLOGICAL  CLASSIFICATION  OF  SOILS 

Weathering  must  be  considered  as  affecting  soils,  whether 
they  are  in  motion  or  at  rest.  This  gives  rise  to  two  general 
classes  of  soil  materials — those  that  have  not  been  shifted 
far  from  their  original  situation  and  those  that  have  suffered 
considerable  translocation.  These  two  general  groups,  desig- 
nated as  sedentary  and  transported,  are  subject  to  subdivision 
as  follows : x 

o  ,1     *        /Residual 
Sedentary  jCumulose 

Gravity Colluvial 

f  Alluvial 

Water  . . . .  i  Marine 

[  Lacustrine 

Ice Glacial 

Wind ^Eolian 


Transported 


25.  Residual  soils.2 — This  group  of  soils  covers  wide 
areas  of  arable  regions,  especially  in  the  tropics  and  sub- 
tropics,  and  comes  from  many  kinds  of  rock.  Residual  soils 
are,  in  the  main,  old  soils,  usually  the  oldest  with  which  we 
deal  in  agricultural  operations,  although  some  residual  soils 
are  comparatively  young.     Since  they  are  formed  in  situ, 

1  Merrill,  G.  P.,  Bocks,  Bock  Weathering  and  Soils,  p.  288;  New  York, 
1906. 

'  For  a  full  discussion  of  the  origin  and  characteristics  of  the  soils 
of  the  United  States  see  Marbut,  C.  F.  et  al.,  Soils  of  the  United  States; 
U.  S.  Dept.  Agr.,  Bui.  96.  1913.  For  the  soils  of  the  Southern  States, 
consult  Bennett,  H.  H.,  The  Soils  and  Agriculture  of  the  Southern 
States;  New  York,  1921. 

38 


GEOLOGICAL  CLASSIFICATION  OF  SOILS        39 

the  rocks  that  underlie  them,  if  sound,  often  given  some  clue 
to  the  character  and  composition  of  the  parent  material.1 
Under  such  conditions  the  changes  that  a  rock  undergoes  in 
forming  a  soil  may  be  studied  to  the  best  advantage. 

Kesidual  soils  are  usually  non-stratified  and  present  a 
heterogeneous  mass  of  material,  grading  from  a  true  soil, 
with  its  normal  content  of  organic  matter,  downward  through 
the  typical  soil  material  to  the  unweathered  country  rock 
below.  Since  such  soil  has  been  subject  to  leaching  for  long 
periods,  a  very  large  amount  of  its  soluble  materials  have 
been  washed  out,  tending  to  leave  high  percentages  of  the 
persistent  elements,  such  as  silica,  iron  and  aluminum.  The 
preceding  discussion  of  soil  formation  has  already  emphasized 
this  phase  sufficiently. 

The  great  age  of  residual  soils  has  given  opportunity  for 
very  thorough  oxidation,  so  that  much  of  the  iron  has  changed 
to  hematite  or  to  the  hydrated  limonite  group.  The  yellow 
color  of  the  latter  group  is  indicative  of  greater  age  than  the 
former.  Since  almost  all  soil  material  contains  considerable 
iron  the  prevailing  colors  of  residual  soils  are  reds  and  yel- 
lows, depending  on  the  degree  of  oxidation  and  hydration. 
Grays,  browns,  and  blacks  often  occur,  however,  where  oxida- 
tion has  not  progressed  or  where  organic  matter  is  present  in 
amounts  sufficient  to  mask  the  iron  coloration. 

As  residual  soils  have  been  subjected  to  intense  physical 
and  chemical  weathering,  the  particles  have  been  reduced 
to  a  very  fine  state  of  division.  Over  residual  areas  the 
heavier  types,  such  as  silt  loams,  clay  loams  and  clays  pre- 
dominate. Sands  and  sandy  loams  may  occur,  however, 
when  the  parent  rock  carried  considerable  quartz  and  a  low 
percentage  of  clay-producing  minerals,  such  as  feldspar,  horn- 
blende   and   augite.      Soils    from    limestones,    granites,    and 

1  Residual  soils  are  not  always  derived  from  rock  similar  to  that 
directly  underlying  the  soil  as  is  often  assumed.  When  the  present 
bed  rock  is  much  different  from  the  stratum  which  gave  rise  to  the 
soil,  the  soil  is  said  to  be  "  inherited.  * ' 


40  NATURE  AND  PROPERTIES  OF  SOILS 

gneiss  1  are  generally  clayey  in  nature,  although  loams  and 
even  stony  loams  may  occur  if  the  limestone  was  sandy  or 
cherty  and  if  the  igneous  rocks  carried  much  quartz.  Dolo- 
mites weather  more  slowly  than  limestone  and  often  give 
rise  to  gravelly  and  stony  types.  Sandstone  of  course  pro- 
duces sandy  soils,  although  a  soil  from  an  argillaceous  sand- 


Fig.  8. — Diagram  showing  the  relationship  between  the  underlying  rocks 
and  the  overlying  residual  soils.    Gettysburg,  Pa.     (After  Emerson.) 


stone  may  be  rather  heavy.  Quartzite  and  slaty  soils  are 
generally  shallow,  and  unfavorable,  both  in  texture  and  fer- 
tility, for  crop  growth.  Soils  from  basic  igneous  rocks,  such 
as  diorite  and  basalt,  generally  produce  sticky  reddish  or  yel- 
lowish clays  containing  little  quartz.  Rocks  that  carry  con- 
siderable mica,  such  as  schists,  give  rise  to  highly  micaceous 


1  For  a  complete  discussion  of  the  influence  of  various  parent  rocks  on 
the  resultant  residual  soil  see  Emerson,  H.  L.,  Agricultural  Geology, 
Chap.  IV;  New  York,  1920. 


GEOLOGICAL  CLASSIFICATION  OF  SOILS        41 

The  following  analyses  *  show  the  general  chemical  char- 
acter of  surface  residual  soils  and  the  variations  that  may 
be  expected: 

Table  V 


Constituents 

1 

2 

3 

4 

Si02 

66.49 

76.71 

74.33 

70.99 

A1203 

17.11 

12.85 

11.00 

11.39 

Fe203 

7.43 

2.81 

4.64 

4.23 

Ti02 

1.02 

.41 

1.04 

1.28 

CaO 

.36 

.08 

1.13 

.93 

MgO 

.31 

.29 

.69 

1.08 

K20 

.62 

3.26 

1.57 

2.71 

Na20 

.16 

.39 

1.53 

.82 

P206 

.17 

.05 

.16 

.19 

S03 

.07 

.12 

.15 

.34 

Organic  matter 

1.26 

1.78 

1.99 

.93 

The  organic  matter  of  residual  soils  largely  depends,  in 
amount  and  condition,  on  climatic  factors.  If  rainfall  and 
temperature,  for  example,  are  favorable  for  the  rapid  and 
continued  development  of  a  natural  vegetation  the  soil  will 
be  rich  in  humus,  so  rich  at  times  as  to  mask  to  a  certain 
extent  the  red  color  so  characteristic  of  such  soils.  If  plants 
do  not  grow  well  on  this  soil,  however,  it  will  be  low  in  organic 
matter  and  probably  in  poor  physical  condition.  Residual 
soils  vary  greatly  in  their  general  characteristics,  especially  as 
to  crop  productivity. 

Residual  soils  are  of  wide  distribution  in  the  United  States, 
particularly  in  the  eastern  and  central  parts,  although  great 

1  Robinson,  W.  O.,  The  Inorganic  Composition  of  Some  Important 
American  Soils;  U.  S.  Dept.  Agr.,  Bui.  122,  Aug.  1914. 

1.  Cecil  clay,  from  granite  and  gneiss.    Charlotte,  N.  C. 

2.  York  silt  loam,  from  schists.     Bethany,  S.  C. 

3.  Penn  silt  loam,  from  sandstone.     Morristown,  Pa. 

4.  Hagerstown  loam,  from  limestone.     Conshohocken,  Pa. 


42  NATURE  AND  PROPERTIES  OF  SOILS 

areas  are  found  in  the  West  as  well.  A  glance  at  the  soil 
map  of  this  country  shows  four  great  eastern  and  central 
provinces — the  Piedmont  Plateau,  the  Appalachian  Moun- 
tains and  Plateaus,  the  Limestone  Valleys  and  Uplands,  and 
the  Great  Plains  Region.  The  first  three  groups  alone  oc- 
cupy 10  per  cent,  of  the  area  of  the  United  States.  The  age 
of  these  soils  varies  in  the  order  named,  showing  that,  while 
they  are  very  old  as  compared  with  other  soils  yet  to  be  dis- 
cussed, there  may  be  vast  periods  of  geologic  time  between 
their  beginnings.  As  a  matter  of  fact,  there  is  probably  a 
greater  difference  in  age  between  the  soils  of  the  Piedmont 
Plateau  and  those  of  the  Great  Plains  Region  than  has  elapsed 
since  the  latter  were  formed.     (See  Fig.  9.) 

26.  Cumulose  soils. — At  relatively  recent  periods  shal- 
low lakes,  ponds,  and  basins  have  been  formed,  partly  by 
stream  action,  partly  by  marsh  conditions  along  sea  or  lake 
coasts,  or  by  glaciation,  a  common  origin  in  northern  United 
States  and  Canada.  The  highly  favorable  moisture  rela- 
tions along  the  banks  of  such  standing  water  has  encouraged 
the  growth  of  many  plants,  such  as  algae,  mosses,  reeds,  flags, 
grass,  and  even  larger  types  of  vegetation.  These  plants 
thrive,  die  and  fall  down  to  be  covered  by  the  water  in  which 
they  grew.  The  water  shuts  out  the  air,  prohibits  rapid 
oxidation,  and  thus  acts  as  a  partial  preservative.  The  decay 
that  does  go  on  is  largely  through  the  agency  of  fungi  and 
anaerobic  bacteria,  that  break  down  the  tissue,  and  liberate 
certain  gaseous  constituents.  As  the  process  continues  the 
organic  mass  becomes  dark  or  even  black  in  color. 

Accumulations  of  this  nature  are  dotted  over  the  entire 
country.  Their  size  may  vary  from  a  few  acres  to  several 
thousand.  Along  streams  the  old  abandoned  beds  offer  ready 
opportunity  for  the  beginning  of  such  accumulations. 
Marshes  either  salt  or  fresh  often  contain  such  deposits. 
Shallow  basins  produced  by  the  scraping  or  damming  action 
of  glaciers  are  frequently  occupied  by  such  material.    In  the 


Tig.  9. — Map  showing  the  soil  provinces  and  soil  regions  of  the  United  States. 

Valleys  and  Uplands  are  residual.       The  soil   regions  of   western   I 


8  ^rj>  «  '  o 


Hougbtcn 


'ST'1    ,»*f"3*"! 


3 R^kfJr'l  *  K/l»m>^^ 
3K4  Fott  ] 


«rja !  1ta?JW**7'$l.\jMW.  r^U"Vin . 


J^SfA  i     r 


.dcM 


uleston 


))C.H»^r 


LEGEND  . 
Soil  Provinces! 
^  Glacial  Lake  and\ 
River  Terraces 


'Glacial  and         Limestone 
Loessial    Valleys  and  Upland; 


Drleans        Apalaoblcol 


D 


Cedar  K«J8 


Appalachion  Moun      Piedmont 
>       ^erat    tains  and  Plateaus      Plateau 


O      F 


River      Atlantic  and  Gulf 
Flood  Plains      Coastal  Plains 


—J—, ^  X         l*      I  rlood  flams      Coastal  Plan 

■            I 1 
| | 

I V-_ WILLIAMS    EMSBAVINS  CO.,   N£-V    YORK 

85^ 8P TV 


)f  the  Piedmont  Plateau,  Appalachian  Mountains  and  Plateaus  and  the  Limestone 
fetes  contain   soils  of  many  different  origins.   (Bui.    96,  Bur.  Soils.) 


GEOLOGICAL  CLASSIFICATION  OF  SOILS        43 

last  named  case  the  beds  are  more  or  less  independent  of 
topography,  and  may  be  found  on  hillsides  as  well  as  in  lower 
lands. 

Cumulose  materials  may  be  grouped  under  two  heads,  peat 
and  muck.  The  only  difference  is  in  their  state  of  decay. 
In  peat  the  stem  and  leaf  structure  of  the  original  plants 
can  still  be  detected,  and  identification  is  quite  possible.  In 
muck,  however,  the  plant  tissue  has  lost  its  identity  as  such 
and  is  merged  into  a  complicated  and  indefinite  mass  of 
organic  material.1 

The  composition  of  peat  and  muck  may  be  much  altered 
by  the  washing-in  of  mineral  matter.  In  some  cases  the  beds 
may  be  from  90  to  95  per  cent  organic,  while  in  other  cases, 
due  to  this  foreign  material,  the  percentage  may  drop  as  low 
as  20  per  cent  giving  a  black  or  swamp  marsh  mud. 

The  analyses  given  illustrate  the  composition  of  some  rep- 
resentative cumulose  soils.     (See  table  VI,  page  44.) 

Peat  and  muck  are  often  of  large  extent 2  and  become  of 
extreme  value  when  drained,  especially  if  they  are  near  a 
good  market.  They  are  of  peculiar  value  in  trucking  oper- 
ations, being  adapted  to  such  crops  as  onions,  celery,  lettuce, 
and  the  like.  Usually  they  must  not  only  be  provided  with 
drainage,  but  must  also  be  treated  with  fertilizers  carrying 

1  The  term  ' '  muck ' '  is  often  used  interchangeably  with  peat.  Tech- 
nically it  is  best  to  limit  the  former  term  to  those  peats  which  are  very 
thoroughly  decomposed  or  contain  a  high  proportion  of  mineral  matter. 
Chemically  muck  is  often  used  in  reference  to  soils  containing  from  20 
to  50  per  cent,  of  organic  matter,  while  peat  is  confined  to  soils  in  which 
the  amount  of  organic  constituents  is  above  50  per  cent.  According  to 
such  a  definition  most  cumulose  soils  are  peats  instead  of  muck.  The 
term  "muck"  is  so  popular,  however,  that  in  the  United  States  its  use 
will  continue  in  spite  of  the  technical  distinctions  that  have  been 
established. 

aAlway  reports  the  following  figures: 

Germany    5,000,000  acres      Wisconsin     . . .   3,000,000  acres 

Sweden   12,000,000    ' '  Ohio     175,000    < « 

Minnesota    .  . .    7,000,000    "  Canada   22,000,000    " 

Alway,  F.  J.,  Agricultural  Value  and  Reclamation  of  Minnesota  Peat 
Soils;  Minn.  Agr.  Exp.  Sta.,  Bui.  188,  1920. 


44  NATURE  AND  PROPERTIES  OF  SOILS 

Table  VI 

COMPARATIVE    CHEMICAL    COMPOSITION    OF    A    PRODUCTIVE    MIN- 
ERAL   SOIL    AND    CERTAIN    REPRESENTATIVE   PEAT 
AND   MUCK   SOILS. 


Soils 


Representative 
mineral  soil 
Minnesota  peat * 
Minnesota  peat * 
Minnesota  Muck  1 
Minnesota  Muck  * 
Florida  peat 2 
Canadian  peat 3 
German  peat  4 
(Low  lime) 
German  peat  4 
(High  lime) 


Organic 
Matter 

Mineral 
Matter 

N 

P306 

KaO 

5.00 

95.0 

.25 

.15 

1.80 

94.00 

6.0 

1.70 

.16 

.04 

59.00 

40.3 

2.35 

.36 

.17 

50.6 

49.4 

1.92 

.40 

— 

41.4 

58.6 

1.78 

.21 

— 

68.4 

31.6 

2.63 

.20 

.17 

74.3 

25.7 

2.19 

.20 

.16 

97.0 

3.0 

1.20 

.10 

.05 

90.0 

10.0 

2.50 

.25 

.10 

CaO 


.70 

.31 

2.52 


.35 
4.00 


phosphorous  and,  especially,  potash.  It  is  also  a  good  prac- 
tice to  start  vigorous  decay  by  the  applicaton  of  barnyard 
manure,  as  the  nitrogen  carried  by  muck  soils  is  usually  not 
very  readily  available  to  plants.5 

1  Alway,  F.  J.,  Agricultural  Value  and  Beclamation  of  Minnesota  Peat 
Soils;  Minn.  Agr.  Exp.  Sta.,  Bui.  188,  Mar.,  1920. 

"Pickel,  G.  M.,  Muck:  Composition  and  Utilisation;  Fla.  Agr.  Exp. 
Sta.,  Bui.  13,  1891. 

'Kept.  Can.  Exp.  Farms,  1910.    Eept.  of  chemist,  p.  160. 

4  Fleischer,  M.,  Die  Anlage  und  die  Bewirtschaftung  von  Moorwiesen 
und  Moorweiden;  Berlin,  1913. 

5  Publications  regarding  the  practical  utilization  of  peat  and  muck 
lands:  Kobinson,  C.  S.,  Utilisation  of  Muck  Lands;  Mich.  Agr.  Exp. 
Sta.,  Bui.  273.  1914.  Whitson,  A.  K.,  et  al.,  The  Improvement  of  Marsh 
Soils;  Wis.  Agr.  Exp.  Sta.,  Bui.  205.  1914.  Stevenson,  W.  H.,  and 
Brown,  P.  E.,  Improving  Iowa's  Peat  and  Alkali  Soils;  la.  Agr.  Exp. 
Sta.,  Bui.  157.  1915.  Smalley,  H.  E.,  Management  of  Muck  Land 
Farms  in  Northern  Indiana  and  Southern  Michigan;  U.  S.  Dept.  Agr., 
Farmers'  Bui.  761.  1916.  Thompson,  H.  C,  Truck  Growing  on  Peat 
Soils;  Jour.  Amer.  Peat  Soc,  Vol.  II,  No.  3,  pp.  113-125.  1918. 
Alway,  F.  J.,  Agricultural  Value  and  Reclamation  of  Minnesota  Peat 
Soils;  Minn.  Agr.  Exp.  Sta.,  Bui.  188.     Mar.,  1920. 


GEOLOGICAL  CLASSIFICATION  OF  SOILS        45 

In  many  cases  muck  and  peat  are  underlaid  at  varying 
depths  by  a  soft  impure  calcium  carbonate,  called  bog-lime.1 
Such  a  deposit  may  come  from  the  shells  of  certain  of  the 
Mollusca,  which  have  inhabited  the  basin,  or  from  aquatic 
plants,  such  as  mosses,  algae  and  species  of  Chara.  In  car- 
bonated water  these  plants  become  incrusted  with  calcium 
carbonate,  possibly  because  of  their  ability  to  absorb  carbon 
dioxide,2  thus  precipitating  the  carbonate.  In  most  cases 
this  carbonate  accumulation  is  due  to  a  combination  of  the 
two  agencies.  Such  material,  because  of  its  richness  in  cal- 
cium, is  valuable  as  a  soil  amendment,  and  often,  where  it  is 
found  pure  enough  in  quality  and  in  sufficiently  large  quan- 
tities, it  is  handled  commercially. 

27.  Colluvial  soils. — This  soil  is  formed  in  regions  of 
precipitous  topography,  and  is  made  up  of  fragments  of  rocks 
detached  from  the  heights  above  and  carried  down  the  slopes 
by  gravity.  Talus  slopes,  cliff  debris,  and  other  heterogeneous 
rock  detritus  are  examples  of  colluvial  soil.  Avalanches  are 
made  up  largely  of  such  material. 

As  the  physical  forces  of  weathering  are  most  active  in  the 
formation  of  these  soils  the  amount  of  solution  and  oxidation 
is  usually  small.  The  upper  part  of  the  accumulation  ex- 
hibits physical  action  to  the  greatest  extent,  the  particles  being 
angular,  coarse,  and  comparatively  fresh;  farther  down  the 
slope  the  material  may  merge  by  degrees  into  ordinary  soil.3 
Such  soils  are  usually  shallow  and  stony,  and  approach  the 
original  rock  in  color  unless  large  amounts  of  organic  matter 
have  accumulated.    Colluvial  soils  are  not  of  great  importance 

1  Bog-lime  is  often  spoken  of  as  marl.  Marl,  as  used  by  the  geologist, 
refers  to  a  calcareous  clay  of  variable  composition.  Bog-lime,  when  it 
contains  numerous  shells,  is  often  termed  shell-marl.  See  Stewart,  C.  F., 
The  Definition  of  Marl;  Econ.  Geol.,  Vol.  4,  No.  5,  pp.  485-489,  1909. 

2  CaH2(C03)2  ?±  C02  +  H2G  +  CaC'O,. 

'Colluvial  soils  generally  merge  so  gradually  into  alluvial  fans  that 
the  line  of  separation  is  difficult  to  establish.  When  the  area  of  col- 
luvial material  is  small,  as  it  usually  is,  it  is  best  included  in  the  fan 
soils. 


46  NATURE  AND  PROPERTIES  OF  SOILS 

agriculturally  because  of  their  small  area,  their  inaccessibility, 
and  their  unfavorable  physical  and  chemical  characteristics. 

28.  Alluvial  soils.1 — In  considering  water  as  a  soil-form- 
ing agency,  it  was  found  to  have  both  cutting  and  transporting 
powers.  Alluvial  soils  are  the  direct  result  of  these  activities, 
especially  the  latter.  The  carrying  power  of  water  varies  di- 
rectly as  the  sixth  power  of  the  velocity ;  so  that  doubling  the 
velocity  increases  the  transportive  ability  sixty-four  times. 
Obviously  any  checking  of  a  stream's  velocity  will  force  it  to 
deposit  its  load,  the  larger  particles  first  and  the  finer  as  the 
current  becomes  more  sluggish.  With  changes  of  velocity  dif- 
ferent grades  of  material  are  laid  down,  giving  rise  to  strati- 
fication, one  of  the  important  characteristics  of  an  alluvial 
soil.  Streams  never  deposit,  either  along  their  course  or  at 
their  delta,  all  of  their  sediment.  Many  tons  of  material  both 
in  suspension  and  in  solution  are  discharged  yearly  into  the 
ocean.2 

There  are  three  general  classes  of  alluvial  soils:  (1)  flood 
plain  deposits,  (2)  deltas,  and  (3)  alluvial  fans.  As  the 
outlet  of  a  stream  is  approached,  its  gradient  generally  be- 
comes less  inclined  and  its  current  is  slackened.  In  a  large 
stream  this  often  means  an  aggrading  of  the  channel  due  to 
the  deposited  material.  A  stream  on  a  gently  inclined  bed 
usually  begins  to  swing  from  side  to  side  in  long,  gentle 
curves,  depositing  alluvial  material  on  the  inside  of  the  curves 
and  cutting  on  the  opposite  banks.    This  results  in  oxbows  and 

1  If  a  detailed  discussion  regarding  alluvial,  marine,  and  glacial  activi- 
ties is  desired,  the  following  books  will  be  helpful:  Tarr,  E.  S.,  and 
Martin,  L.  M.,  College  Physiography ;  New  York,  1918.  Pirsson,  L.  V., 
A  Text  Book  of  Geology,  Part  I;  New  York,  1915.  Emerson,  H.  L., 
Agricultural  Geology;  New  York,  1920.  The  considerations  of  river 
and  stream  action  by  Eussell  and  Davis  are  classical:  Kussell,  I.  C, 
Rivers  of  North  America;  New  York,  1898.  Davis,  W.  M.,  The  Eivers 
and  Valleys  of  Pennsylvania;  Geological  Essays,  Boston,  1909. 

"Streams  each  year  discharge  into  the  ocean,  on  the  average,  100  tons 
of  soluble  matter  for  every  square  mile  of  drainage  area.  The  Mississippi 
River  pours  into  the  Gulf  of  Mexico  each  year  406,250,000  tons  of  sedi- 


GEOLOGICAL  CLASSIFICATION  OF  SOILS       47 

lagoons,  which  are  ideal  not  only  for  the  further  deposition 
of  alluvial  matter  but  also  for  the  formation  of  cumulose  soils. 
This  state  of  meander  naturally  increases  the  probability  of 
overflow  in  high  water,  a  time  when  the  stream  is  carrying 
much  suspended  matter.  This  suspended  material  is  deposited 
over  the  flooded  areas;  the  coarser  near  the  channel,  building 
up  natural  levees;  the  finer  sediment  farther  away  in  the 
lagoons  and  slack  water. 

Due  to  a  change  in  grade,  a  stream  may  cut  down  through 
its  already  well-formed  alluvial  deposits,  leaving  terraces  on 
one  or  both  sides.  Often  two,  or  even  three,  terraces  may  be 
detected  along  a  valley,  marking  a  time  when  the  stream-bed 
was  at  these  elevations.  On  the  lower  slopes  of  hills  bordering 
valleys  the  colluvial  deposits  may  touch  or  even  mingle  with 
the  alluvial,  furnishing  the  stream  with  some  detritus.  Flood 
plain  soils  are  variable  in  character,  ranging  from  sandy  loams 
to  heavy  clays. 

A  great  deal  of  the  sediment  carried  by  streams  is  not  de- 
posited in  the  flood  plain  but  is  discharged  into  the  body  of 
water  to  which  the  stream  is  tributary.  Unless  there  is  suffi- 
cient current  and  wave  action  the  suspended  material  ac- 
cumulates, forming  a  delta.  Such  deposits  are  by  no  means 
universal,  being  found  at  the  mouths  of  but  a  small  propor- 
tion of  the  rivers  of  the  world.  A  delta  is  generally  a  con- 
tinuation of  the  flood  plain  and  as  it  is  built  farther  and  far- 
ther out  the  stream  is  forced  to  aggrade  its  bed  and  both 
flood  plain  and  delta  are  raised.  Near  the  front  of  the  delta 
the  land  is  swampy ;  farther  back  it  is  higher  and  may  assume 
considerable  agricultural  importance. 

Where  streams  descend  from  mountains  or  plateaus,  sudden 
changes  in  gradient  often  occur  as  the  stream  emerges  in  the 
lower  lands.  A  deposition  of  sediment  is  thereby  forced,  giv- 
ing rise  to  alluvial  fans.1     They  differ  from  deltas  in  their 

1  As  already  noted,  alluvial  fans  and  colluvial  material  are  very  closely 
related.  Soil  survey  classifications  usually  do  not  recognize  the  latter 
separation. 


48 


NATURE  AND  PROPERTIES  OF  SOILS 


location  and  in  the  character  of  their  material.  The  material 
of  the  latter  is  generally  sandy,  more  or  less  porous  and  well 
drained.    Deltas,  on  the  other  hand,  are  characterized  by  poor 

drainage  and  by  heavy 

soils,  silt  loams,  clay 
loams  and  clays  predom- 
inating. 

Alluvial  soils,  espe- 
cially those  of  flood 
plain  origin,  are  com- 
paratively young.  Delta 
and  first  bottom  soils  are 
usually  in  need  of  drain- 
age. Alluvial  fans  and 
terrace  soils  are  olten 
loose  and  open  to  the 
point  of  droughtiness. 
The  latter  group  is  usu- 
ally not  so  well  sup- 
plied with  organic  mat- 
ter as  are  the  delta  and 
flood  plain  soils,  which 
exist  under  conditions 
where  organic  accumu- 
lation is  rapid.  All  allu- 
vial soils  are  greatly  in- 
fluenced by  the  source 
of  the  detritus.  For  ex- 
ample, a  red  upland  soil 
will  give  a  reddish  allu- 
vial, while  a  soil  or  rock  poor  in  lime  will  certainly  not  be 
parent  to  one  rich  in  that  constituent.  Alluvial  soils  are  gen- 
erally richer  in  the  essential  constituents  than  the  soils  from 
which  they  are  a  wash,  as  is  shown  by  the  following  data 
from  North  Carolina. 


Fig.  10. — The  flood-plain  and  delta  of  the 
lower  Mississippi  Kiver. 


GEOLOGICAL  CLASSIFICATION  OF  SOILS       49 
Table  VII 

CHEMICAL  ANALYSES  OF  TWO  ALLUVIAL  SURFACE  SOILS  AND  THE 

RESPECTIVE   SURFACE  SOILS  FROM   WHICH 

THEY  WERE  DERIVED.1 


Constituents 

l 

Alluvial 

2 
Upland 

3 

Alluvial 

4 
Upland 

N 

.073 

.076 

1.697 

1.103 

.048 

.041 

1.330 

.200 

.173 
.118 
.433 
.417 

.038 

P205 

.027 

K20 

.286 

CaO 

.221 

Delta  soils,  where  they  occur  in  any  acreage,  are  very  im- 
portant. The  deltas  of  the  Mississippi,  Ganges,  Po,  Tigris, 
and  Euphrates  rivers  are  striking  examples.  Egypt,  for 
centuries  the  granary  of  Rome,  bespeaks  the  fertility  of  such 
land.  Flood  plain  soils  are  found  to  a  certain  extent  along 
every  stream,  the  greatest  development  in  the  United  States 
occurring  along  the  Mississippi.  This  area  varies  from  forty 
to  sixty  miles  in  width  and  has  a  length  from  Cairo  to  the 
Gulf  of  over  600  miles.  Such  soils  are  very  rich  but,  if 
they  are  first  bottoms,  they  require  drainage  and  protection 
from  overflow.  Alluvial  fan  soils  are  found  over  wide  areas 
in  arid  and  semi-arid  regions  and  when  irrigated  and  prop- 
erly handled  have  proven  very  productive.  They  often  occur 
in  large  enough  areas  in  humid  regions  to  be  of  considerable 

1  Williams,  C.  B.,  et  al.,  Report  on  the  Piedmont  Soils ;  Bui.  N.  C. 
Dept.  Agr.,  Vol.  36,  No.  2,  Feb.,  1915. 

1.  Average    of    8    analyses    of    Piedmont    alluvial    soils,    Congaree 
series,  to  a  large  extent  a  wash  from  the  Cecil. 

2.  Average  of  71  analyses  of  Cecil  series  soils,  the  typical  upland 
soil  of  the  North  Carolina  Piedmont. 

Williams,  C.  B.,  et  al.,  Report  on  the  Coastal  Plain  Soils;  Bui.  N.  C. 
Dept.  Agr.,  Vol.  39,  No.  5,  May,  1918. 

3.  Average  of  8  analyses  of  coastal  plain  alluvial  soils,  Johnston 
and  Kalmia  series. 

4.  Average   of   165   analyses  of   Norfolk  series   soils,   the  typical 
upland  coastal  plain  soil  of  North  Carolina. 


50  NATURE  AND  PROPERTIES  OF  SOILS 

importance.  The  type  of  farming  and  the  crops  grown  on  any 
area  of  alluvial  soil  will  vary  with  climate,  soil  conditions, 
and  markets. 

29.  Marine  soils.1 — A  great  deal  of  the  sediment  carried 
away  by  stream  action  is  eventually  deposited  in  the  sea, 
the  coarser  fragments  near  the  shore,  the  finer  particles  at 
a  distance.    Such  material  is  largely  clastic  and  if  there  have 


Fig.  11. — Block  diagram  showing  how  marine  soils  are  formed  and  their 
relation  to  the  uplands.  The  emerged  coastal  plain  has  already 
suffered  some  dissection  from  stream  action.     (After  Emerson.) 

been  many  changes  in  shorelines,  the  alternating  beds  will 
show  no  regular  sequence.  Such  material,  when  raised  above 
the  sea  by  diatrophism  and  subjected  to  sufficient  weathering 
and  denudation,  is  classed  as  marine  soil.     (See  Fig.  11.) 

Such  material  has  been  worn  and  triturated  by  a  number 
of  agencies.  First,  the  forces  necessary  to  throw  it  into 
stream  suspension  were  active,  and  next  it  was  swept  into 

*For  an  excellent  discussion  of  the  marine  soils  of  the  Atlantic  and 
Gulf  coasts  of  the  United  States  see  Bennett,  H.  H.,  Soils  and  Agri- 
culture of  the  Southern  States;  New  York,  1921. 


GEOLOGICAL  CLASSIFICATION  OF  SOILS        51 

the  ocean  to  be  deposited  and  stratified,  possibly  after  being 
pounded  and  eroded  by  the  waves  for  years.  At  last  came 
the  emergence  above  the  sea  and  the  action  of  the  forces 
of  weathering  in  situ.  The  latter  effects  are  of  great  moment 
since  they  determine  not  only  the  topography  but  the  fertility 
of  the  new  soil  as  well.  The  availability  of  the  nutritive  ele- 
ments, and  especially  the  amounts  of  organic  matter,  are  de- 
termined by  recent  and  still  active  forces. 

The  marine  soils  of  the  United  States,  while  younger  than 
most  of  our  residual  soils,  are  usually  more  worn  and  gener- 
ally carry  less  of  the  nutrient  elements.  Their  silica  con- 
tent is  very  high  and  they  are  often  sandy,  especially  along 
the  Atlantic  seaboard.  Sands,  sandy  loams,  and  loams  pre- 
dominate, although  silt  loams  and  clays  are  by  no  means  un- 
usual, especially  in  the  Atlantic  and  Gulf  coastal  flatwoods 
and  the  black  prairies  and  interior  flatwoods  of  Alabama  and 
Mississippi.  The  organic  content  of  the  sandy  soils  is  gen- 
erally low,  but  on  the  heavier  types  it  may  almost  equal  delta 
and  flood  plain  soils. 

A  direct  comparison 1  between  typical  coastal  plain  and 
residual  soils  usually  shows  the  former  to  be  considerably 
higher  in  silica  but  lower  in  iron  and  aluminium.  The  marine 
soil  is,  on  the  other  hand,  lower  in  phosphoric  acid  and 
potash.  The  nitrogen,  organic  matter,  and  lime  are  so  vari- 
able in  both  soils  that  no  reliable  deductions  can  be  drawn. 
The  following  data  from  Eastern  United  States  substantiate 
the  above  generalizations.2     (Tables  VIII  and  IX,  page  52.) 

The  soils  of  the  Atlantic  and  Gulf  coastal  provinces,  formed 
as  vast  outwash  plains  and  occupying  11  per  cent,  of  the  area 
of  the  United  States,  are  very  diversified,  due  to  source  of 

1  When  soils  are  compared  on  the  strictly  chemical  basis  great  caution 
should  be  observed  in  drawing  conclusions  as  to  relative  productivity. 
The  amount  of  a  nutrient  present  is  by  no  means  a  measure  of  its 
availability.  A  chemical  analysis  usually  throws  but  little  light  on  the 
fertilizer  needs  of  a  soil. 

2  See  also  Walker,  S.  Sv  Chemical  Composition  of  Some  Louisiana 
Soils  as  to  Series  and  Texture;  La.  Agr.  Exp.  Sta.,  Bui.  177,  Aug.,  1920. 


52 


NATURE  AND  PROPERTIES  OF  SOILS 


Table  VIII 

COMPARATIVE    COMPOSITIONS    OF    COASTAL    PLAIN    AND    RESIDUAL 
SOILS  OF  EASTERN  UNITED  STATES.1 


Constituents 


Si02. 
Ti02. 
A1203 
Fej2Oa 


2 
Kesidual 


77.72 

.90 

9.13 

3.75 


Table  IX 


COMPARATIVE  COMPOSITIONS  OF  NORTH  CAROLINA  COASTAL  PLAIN 
AND  RESIDUAL  SOILS.2 


Constituents 

l 
Costal  Plain 

2 
Coastal  Plain 

3 

Kesidual 

N 

.038 
.027 
.286 
.221 

.138 
.033 
.346 
.394 

.048 

P90R 

.041 

K20 

1.330 

CaO 

.200 

Robinson,  W.  O.,  et  ah,   Variation  in  the  Chemical  Composition  of 
Soils;  U.  S.  Dept.  Agr.,  Bui.  551,  June,  1917. 

1  and  2.     Average  analyses  of  15  coastal  plain  and  8  residual  soils, 
respectively,  taken  from  various  places  in  eastern  United  States. 

2  Williams,   C.   B.,   et  ah,   Beport   on   the   Coastal   Plain   Soils;    Bui. 
N.  C.  Dept.  of  Agr.,  Vol.  39,  No.  50,  May,  1918. 

1.  Average  of  165  analyses  of  Norfolk  series  soils.  This  is 
the  typical  soil  of  the  Atlantic  coastal  plain. 

2.  Average  of  84  analyses  of  Portsmouth  series  soils.  This 
series  is  above  the  average  coastal  plain  soil  in  organic 
matter    and    fertility. 

Williams,  C.   B.,   et  ah,  Beport  on  the  Piedmont  Soils;   Bui.   N.   C. 
Dept.  of  Agr.,  Vol.  36,  No.  2,  Feb.,  1915. 

3.  Average  of  71  analyses  of  Cecil  series  soils.  This  series  is 
the  typical  residual  soil   of  the  Piedmont  plateau. 


GEOLOGICAL  CLASSIFICATION  OF  SOILS        53 

material,  age,  and  climatic  conditions.  There  are  great 
tracts  of  general  farming  land,  besides  wide  areas  of  special- 
purpose  soils  adapted  to  highly  specialized  industries.  The 
latter  soils  require  refined  and  intensive  methods  of  culti- 
vation. Except  for  certain  areas  the  coastal  plain  soils  are 
well  aerated  and  easy  to  cultivate.  Except  in  the  lower 
coastal  plain  belt  they  are  well  drained.  Severe  leaching  as 
well  as  serious  erosion  occurs  in  times  of  heavy  rainfall. 
When  sufficiently  supplied  with  organic  matter,  carefully 
fertilized,  and  cultivated  properly,  these  soils  support  a  great 
variety  of  crops  such  as  cotton,  maize,  oats,  forage  crops, 
and  peanuts,  besides  vegetables  and  fruits  of  many  varieties. 
30.  The  ice  age  and  the  American  ice  sheet.1 — If  in  any 
region  the  temperature  and  snowfall  stand  in  such  rela- 
tionship that  the  heat  of  summer  does  not  offset  the  winter's 
accumulation  of  snow,  great  snowfields  form.  If  this  con- 
dition persists  year  after  year  the  temperature  is  reduced 
to  such  an  extent  as  to  increase  the  proportion  of  the  snowfall, 
which  escapes  the  summer  heat.  The  pressure  of  overlying 
snow  and  the  influence  of  the  summer  melting  soon  change 
the  snow  into  ice  with  a  complicated  recrystallization.  As  the 
depth  of  the  accumulation  increases  outward  movement  is 
inaugurated  due  to  the  strong  lateral  pressure.  As  the  ice 
moves  slowly  forward  under  this  tremendous  pressure,  with 
an  almost  incredible  thickness  and  a  plasticity  which  ordi- 
nary ice  does  not  possess,  it  conforms  itself  to  the  uneven- 
ness  of  the  areas  invaded.  It  rises  over  hills  and  shapes 
itself  to  valleys  with  surprising  ease.  Not  only  is  the  exist- 
ing soil  mantle  swept  away  by  such  an  invasion  but  the 
underlying  rocks  are  ground  and  gouged.  When  the  ice 
melts  back  and  the  region  is  again  free  a  mantle  of  soil  ma- 
terial remains. 

*For  a  complete  discussion  of  glaciers  and  glaciation,  see  Salis- 
bury, R.  D.,  The  Glacial  Geology  of  New  Jersey;  Geol.  Survey  of 
New  Jersey,  Vol.  5,   1902. 


54  NATURE  AND  PROPERTIES  OF  SOILS 

This  drift  is  often  merely  ground-up  rock,  at  other  times 
the  original  soil  is  mixed  with  foreign  detritus,  while  again 
the  variable  mixtures  may  be  wholly  reworked  and  consid- 
erably stratified.  Besides  this  the  streams  of  water,  which 
issue  from  under  the  ice,  may  be  instrumental  in  distribut- 
ing sediments  for  miles  beyond  the  ice  front.  Glacial  lakes, 
when  in  existence  for  sufficiently  long  periods,  furnish  basins 
for  the  deposition  of  materials  derived  from  the  erosive  and 
grinding  influence  of  the  ice.  The  ice  may  also  provide  a 
large  amount  of  detritus  so  fine  as  to  be  susceptible  to  wind 
movement,  and  thus  aeolian  influences  as  well  as  alluvial  and 
lacustrine  may  be  concomitant  to  a  great  ice  invasion. 

During  the  Pleistocene  northern  North  America,  as  well 
as  part  of  Europe,  was  successively  invaded  by  ice  sheets, 
which  exerted  the  influences  above  described  and,  while  the 
central  ice  caps  in  Canada  probably  never  wholly  disap- 
peared, the  regions  to  the  southward  certainly  experienced 
alternate  glaciation  and  inter glaciat ion.  At  least  five  in- 
vasions are  evident  in  central  United  States.  Debris  from  the 
last,  called  the  Wisconsin,  now  covers  wide  areas.  The  in- 
terglacial  periods  are  shown  by  forest  beds,  accumulations 
of  organic  matter,  and  evidences  of  erosion  between  the  drift 
deposited  by  the  successive  ice  sheets.  Some  of  the  inter- 
glacial  periods  evidently  were  times  of  warm,  and  even  semi- 
tropical,  climate.  Just  what  was  the  exact  cause  of  the  ice 
age  is  still  under  dispute.1  That  it  was  due  to  a  change  in 
the  carbon  dioxide  content  of  the  air  seems  as  probable  as 
any  of  the  numerous  hypotheses  that  ha\e  been  advanced. 

The  area  covered  by  glaciers  in  North  America  is  estimated 
as  4,000,000  square  miles,  while  at  least  20  per  cent,  of  the 
United  States  is  either  directly  or  indirectly  influenced  by  the 
debris.    The  greatest  southward  extension  of  the  ice  is  marked 

1  Humphreys,  W.  J.,  Factors  of  Climatic  Control,  Jour.  Franklin  Inst., 
Vol.  189,  No.  1,  pp.  63-98,  Jan.,  1920. 


GEOLOGICAL  CLASSIFICATION  OF  SOILS        55 

by  a  terminal  moraine  wherever  the  ice  margin  was  station- 
ary long  enough  to  permit  such  an  accumulation.  Many 
other  moraines  are  found  to  the  northward,  marking  points 
where  the  ice  became  stationary  for  a  time  as  it  retreated 
by  melting.1  While  the  moraines  are  generally  outstand- 
ing topographic  features,  they  are  commonly  unimportant 
agriculturally  due  to  their  small  area  and  unfavorable  physi- 
ography. The  ground  moraine  is  the  material  which  fur- 
nishes the  bulk  of  the  soils  which  have  directly  resulted  from 
glaciation.  This  ground  moraine  is  of  wide  extent  and  pos- 
sesses a  favorable  agricultural  topography.  The  weathering 
in  situ  of  this  great  area  of  soil  material  has  evolved  one  of 
the  most  productive  soil  provinces  of  the  world. 

31.  Glacial  soils. — The  soils  which  have  been  developed 
from  the  glacial  till  are  usually  rather  heavy,  loams,  silt 
loams,  and  clay  loams  predominating.  The  subsoil  is  gen- 
erally finer  than  the  surface  and  may  induce  poor  drainage. 
The  individual  particles  of  such  soils  are  less  weathered  than 
those  of  residual  soils.  The  feldspars  have  retained  their 
normal  luster  and  the  iron  staining  so  common  in  the  Pied- 
mont Plateau  is  almost  absent.  The  color  is  usually  sub- 
dued, grays  and  browns  prevailing.  Red  glacial  soil  may 
occur,  however,  where  red  sandstones  have  been  ground  up 
or  where  considerable  residual  soil  has  been  incorporated 
in  the  till.  The  subsoils  usually  present  colors  ranging  from 
light  gray  and  yellows  to  brown.  Mottling  is  common,  es- 
pecially in  the  subsoil,  due  to  lack  of  aeration. 

The  chemical  composition  of  glacial  soils  approaches  that 
of  the  parent  rock  more  nearly  than  does  any  other,  since 

1  The  position  of  the  ice  front  of  a  glacier  is  determined  by  the 
relationship  between  the  forward  movement  of  the  ice  and  the  rate 
of  melting.  When  the  former  is  dominant,  the  ice  front  advances. 
When  melting  in  dominant,  the  ice  front  recedes.  When  these  two 
forces  are  balanced,  conditions  are  favorable  for  a  stand  of  the 
ice  and  the  building  of  a  moraine. 


56 


NATURE  AND  PROPERTIES  OF  SOILS 


the  forces  of  weathering,  while  they  have  had  time  to  pro- 
duce a  soil  from  the  material  left  by  the  ice,  have  not  as  yet 
seriously  depleted  the  essential  constituents.  The  mineral 
elements  in  such  soils  are  governed  to  a  considerable  degree 
by  the  composition  of  the  original  rock.     Calcium  content, 


Fig.  12. — Block  diagram  showing  the  relationship  which  sometimes  exists 
between  glacial  soils  and  the  underlying  rocks.  Glacial  movement 
left  to  right.      (After  Emerson.) 


for  example,  is  controlled  largely  by  such  a  relationship. 
The  hill  soils  of  southern  New  York  (Volusia  and  Lords- 
town)  come  from  shales  low  in  lime  and  their  productive- 
ness is  seriously  affected  thereby.  On  the  other  hand,  cer- 
tain glacial  soils  of  central  New  York  and  of  the  Mississippi 
Valley  (Ontario  and  Miami)  have  been  formed  from  cal- 
careous till  and  owe  their  productivity  partly  thereto.     Gla- 


GEOLOGICAL  CLASSIFICATION  OF  SOILS        57 

cial  soils  from  limestones  generally  contain  plenty  of  lime, 
a  condition  that  is  far  from  true  with  residual  soils.1 

The  organic  content  of  glacial  soils  depends  to  a  large 
extent  on  the  climatic  conditions  under  which  the  soil  has 
existed  since  its  formation.  If  environmental  factors  have 
been  such  as  to  encourage  the  accumulation  of  organic  mat- 
ter, these  soils  will  exhibit  the  deep  black  color  that  arises 
from  the  presence  of  such  material.  If,  however,  conditions 
do  not  encourage  the  natural  growth  of  a  heavy  vegetation, 
the  amount  of  organic  matter  in  such  virgin  soil  will  be  low. 
Lime  and  other  nutritive  elements  may  also  be  a  great  factor 
in  the  development  of  vegetation  on  these  soils.  Glacial  till 
soils  are  distributed  over  all  the  area  north  of  the  great 
terminal  moraine,  and  stretch,  roughly,  from  New  Eng- 
land to  the  Pacific  coast.  They  comprise  a  great  variety  of 
soils,  differing  not  only  in  their  physical  characters,  but  also 
as  to  fertility.  They  are  adapted  to  many  crops,  but  general 
farming  is  practiced  on  them  to  the  greatest  degree.  This 
means  extensive,  rather  than  intensive,  operations.    In  some 

1  Partial  Analyses   of  Soils   from   the  Limestone   Driftless   and 
Glacial  Region  of  Wisconsin  2  Are  of  Interest  in  This  Regard  : 


Constituents 


Residual 


Glacial 


Si02 

A1203  +  Fe20 

MgO    

CaO   

K2G 

P20B 

COa    


71.13 
18.02 
.38 
.85 
1.61 
.02 
.43 


49.13 

31.12 

1.92 

1.22 

1.61 

.04 

.39 


40.22 
11.30 

7.80 
15.65 

2.36 

.05 

18.76 


48.81 
10.07 

7.95 
11.83 

2.60 

.13 

15.47 


2  Chamberlain,  T.  C.  and  Salisbury,  R.  D.,  The  Driftless  Area  of  the 
Upper  Mississippi;  Sixth  Ann.  Rep.  U.  S.  Geol.  Survey,  pp.  249-250,  1885. 
These  analyses  illustrate  to  very  good  advantage  the  beliefs  entertained 
by  Chamberlain  and  Salisbury  regarding  the  differences  between  residual 
and  glacial  clays.  Residual  clay  is  designated  by  them  as  "rock  rot," 
and  glacial  clay  as  ' '  rock  flour. ' '  The  latter,  being  less  weathered,  "re- 
tains a  larger  proportion  of  its  easily  soluble  materials. 


58  NATURE  AND  PROPERTIES  OF  SOILS 

regions  dairying  has  been  developed  to  a  large  extent,  while 
in  certain  localities,  where  climate,  soil,  and  market  are  fa- 
vorable, trucking  is  of  great  importance. 

32.  Effect  of  glaciation  on  agriculture.1 — In  comparing 
glaciated  soils  with  corresponding  residual  areas,  certain 
differences  are  usually  apparent.  The  agricultural  condition 
within  the  zone  of  glaciation  is  usually  consistently  higher 
than  that  beyond  the  regions  of  drift  accumulation.  The 
extensive  leveling  due  to  glacial  erosion  and  deposition  has 
almost  always  resulted  favorably  for  agricultural  operations. 
Even  the  thickness  of  the  drift  is  found  to  conserve  the 
ground  water  supply.  While  it  is  difficult  to  show  any  con- 
sistent difference  between  residual  and  glacial  soils  as  to  total 
constituents,  it  is  generally  admitted  that  glaciation  has  been 
a  benefit  to  agriculture,  in  that  the  soils  have  been  rejuven- 
ated and  their  crop-producing  power  raised. 

The  dominant  textural  quality  of  glacial  soils  seems  adapted 
to  certain  staple  food  crops,  and,  due  to  their  interming- 
ling, a  considerable  opportunity  for  diversified  and  intensi- 
fied farming  is  offered.  It  is,  therefore,  evident  that  in  any 
study  of  soils,  particularly  those  of  the  United  States,  a 
careful  consideration  of  the  effects  of  glaciation  is  neces- 
sary. Even  the  alterations  in  topography  are  factors  not 
to  be  ignored.  In  a  comparison  of  the  driftless  area  of  Wis- 
consin with  the  glaciated  parts  only  43  per  cent,  of  the 
former  is  improved  as  against  61  per  cent-,  of  the  latter,  while 
the  value  of  the  farms  on  the  glaciated  soil  averages  50  per 
cent  higher.  The  same  general  differences  appear  between 
the  glacial  and  residual  soils  of  Indiana  and  Ohio. 

33.  Lacustrine  soils — glacial  lake. — Great  torrents  of 
water  were  constantly  gushing  from  the  front  of  the  great 

1  Whitbeck,  E.  H.,  The  Glaciated  and  Driftless  Portions  of  Wisconsin; 
Bui.  Geog.  Soe.  Phil.,  Vol.  IX,  No.  3,  pp.  10-20,  1911.  Von  Englen, 
O.  D.,  Effects  of  Continental  Glaciation  on  Agriculture;  Bui.  Amer. 
Geog.  Soc,  Vol.  XL VI,  pp.  353-355,  1914.  Ames,  J.  W.,  and  Gaither, 
E.  W.,  Soil  Investigations;  Ohio  Agr.  Exp.  Sta.,  Bui.  261,  1913. 


GEOLOGICAL  CLASSIFICATION  OF  SOILS        59 


ice  sheets  as  they  advanced  and  retreated  in  response  to  their 
environment.  The  great  loads  of  sediment  carried  by  such 
streams  were  either  dumped  down  immediately  or  carried  to 
other  areas  for  deposition.  As  long  as  the  water  had  ready 
egress  it  flowed  rapidly  away  to  deposit  its  load  as  gravelly 


Fig.  13. — Diagram  showing  how  glacial  lakes  were  formed  in  New  York 
State.  The  lighter  shading  represents  the  Ontarian  ice  lobe;  the 
darker  shading  indicates  the  position  of  the  glacial  lake  waters  in 
the  Ontario  and  Hudson  river  basins.     (After  Fairchild.) 

outwash,  river  terraces,  valley  trains  and  alluvial  fans.  In 
many  cases,  however,  the  ice  front  came  to  a  stand  where 
there  was  no  such  ready  egress  and  ponding  occurred.  Often 
very  large  lakes  were  formed  which  existed  for  many  years. 
(See  Fig.  13.) 

With  the  ice  melting  rapidly  on  the  hill  tops  these  lakes 
were  constantly  fed  by  torrents  from  above,  which  were 
laden  with  sediment  derived  not  only  from  under  the  ice, 


60  NATURE  AND  PROPERTIES  OF  SOILS 

but  also  from  the  unconsolidated  till  sheet  over  which  it 
flowed.  As  a  consequence  there  were  in  the  glacial  lakes 
deposits  ranging  from  coarse  delta  materials  near  the  shore  to 
fine  silts  and  clay  in  the  deeper  and  stiller  waters.  Such 
materials  now  cover  large  areas,  not  only  in  New  York  state 
and  along  the  Great  Lakes,  but  also  in  the  Red  River  Valley 
and  in  the  valleys  of  the  Rocky  Mountains  and  the  Cascades 
and  Sierra  Nevadas.  They  make  up  by  far  the  most  im- 
portant of  the  lacustrine  soils.  Glacial  lake  soils  probably 
present  as  wide  a  variation  in  physical  characteristics  as  any 
of  the  great  soil  provinces.  Being  deposited  by  water  they 
have  been  subject  to  much  sorting  and  stratification,  and 
range  from  coarse  gravels  on  the  one  hand  to  fine  clays 
on  the  other.  They  are  generally  found  as  the  lowland  soils 
in  any  region,  although  they  may  occur  well  up  on  the 
hillsides  if  the  shores  of  the  old  lakes  encroached  thus  far. 
The  color  of  such  soils  varies  from  gray  to  black,  according 
to  the  degree  of  organic  matter  present.  The  organic  con- 
tent of  such  soils,  as  with  the  glacial  till,  varies  with  climate, 
and  may  be  high,  low,  or  medium  according  to  conditions. 
The  thickness  of  glacial  lake  deposits  is  variable,  ranging 
from  a  few  to  many  feet.  In  chemical  composition  they 
closely  approximate  the  soil  material  from  which  they  were 
derived.  This  is  particularly  true  as  regards  the  presence 
of  lime.  The  distribution  of  glacial  lake  deposits  is  not 
only  wide  but  the  areas  are  large  enough  to  be  of  great 
agricultural  influence.  Extending  westward  from  New 
England  along  the  Great  Lakes  until  the  broad'  expanse 
of  the  Red  River  Valley  is  reached  these  deposits  have  pro- 
duced some  of  the  most  important  soils  of  the  northern 
states.  They  are  valuable  not  only  for  extensive  cropping 
with  grain  and  hay,  but  also  for  fruit  and  trucking. 

34.  Lacustrine  soils — recent  lake. — While  the  glacial 
lake  deposits  were  formed  many  thousands  of  years  ago  the 
lake  soils  of  the  second  group  are  still  in  process  of  construe- 


GEOLOGICAL  CLASSIFICATION  OF  SOILS       61 

tion.  It  is  a  well-known  fact  that  lakes  are  only  enlarged 
stream  beds,  and  are  doomed  ultimately  to  be  filled  by  river 
sediments.  Such  soils  have  been  reclaimed  to  a  certain  ex- 
tent, but  their  acreage  is  not  large  enough  to  give  them  the 
importance  of  the  glacial  lake  soils.  The  lake  soil  is  usually 
of  a  fine  character,  rich  in  organic  matter  and  of  good  tilth. 
If  properly  drained,  it  is  almost  invariably  highly  produc- 
tive, and  is  adapted  to  a  variety  of  crops  depending  on  cli- 
matic conditions. 

35.  .ffiolian  soils. — Loess. — During  glaciation  much  fine 
material  was  carried  miles  below  the  front  of  the  ice  sheets 
by  streams  that  found  their  source  within  the  glaciers.  This 
fine  sediment  was  deposited  over  wide  areas  by  the  over- 
loaded rivers.  The  accumulations  occurred  below  the  ice 
front  at  all  points,  but  seem  to  have  reached  their  greatest 
development  in  what  is  now  the  Missouri  Valley  and  the 
Great  Plains.  Much  of  the  sediment  in  the  latter  area  prob- 
ably came  from  local  glaciers,  which  debouched  from  the 
Rockies. 

It  is  generally  agreed  by  glacialists,  that  a  period  of  aridity, 
at  least  as  far  as  this  particular  region  is  concerned,  im- 
mediately followed  the  retreat  of  the  ice.  The  low  rain- 
fall of  this  period  was  accompanied  by  strong  westerly  winds. 
These  winds,  active  perhaps  through  centuries,  were  instru- 
mental in  the  picking-up  and  distributing  of  this  fine  ma- 
terial over  wide  areas  of  the  Mississippi,  Ohio,  and  Missouri 
valleys.  One  strong  argument  for  this  aeolian  origin  is  that 
the  soil  is  in  its  deepest  and  most  characteristic  development 
along  the  eastern  banks  of  the  large  streams.  Especially 
noticeable  is  the  extension  down  the  eastern  side  of  the  Missis- 
sippi River  almost  to  the  Gulf  of  Mexico.  This  wind-blown 
material,  called  loess,  is  found  over  wide  areas  in  the  United 
States,  in  most  cases  covering  the  original  till  mantle.  It 
covers  eastern  Nebraska  and  Kansas,  southern  and  central 
Iowa  and  Illinois,  northern  Missouri  and  parts  of  Ohio  and 


62  NATURE  AND  PROPERTIES  OF  SOILS 

Indiana,  besides  a  wide  band,  as  already  noted,  extending 
southward  along  the  eastern  border  of  the  Mississippi  River. 
Due  to  its  mode  of  origin,  its  depth  is  always  greatest  near 
the  streams  and  gradually  becomes  less  farther  inland.  In 
places,  notably  along  the  Missouri  and  Mississippi  rivers,  its 
accumulation  has  given  rise  to  great  bluffs,  which  bestow  a 
characteristic  topography  to  the  region. 

Not  only  is  loess  found  over  thousands  of  square  miles  in 
the  central  part  of  the  United  States  but  it  occurs  else- 
where in  large  areas.  It  is  greatly  developed  in  northern 
France  and  Belgium,  and  along  the  Rhine  in  Germany,  where 
it  is  an  important  soil  in  all  the  valleys  that  are  tributary 
to  that  river.  Silesia,  Poland,  southern  Russia,  Bohemia, 
Hungary  and  Roumania  have  deposits  of  this  highly  fertile 
material.  Some  of  the  most  important  moves  of  the  World 
War  had  as  their  aim  the  possession  of  these  fertile  areas. 
In  China  loess  is  found  over  a  very  large  part  of  the  valley 
of  the  Hwangho,  a  region  probably  larger  in  area  than  France 
and  Germany  combined.  The  thickness  of  the  deposit  is 
variable,  ranging  from  a  few  feet  to  several  thousand  in 
places.  The  depth  is  practically  always  sufficient  for  any 
form  of  agricultural  operations. 

Loess  is  usually  a  fine  calcareous  silt  or  clay  loam,  of  a 
yellowish  or  yellowishy  buff  color.  While  it  may  be  readily 
pulverized  when  subjected  to  cultivation,  it  possesses  remark- 
able tenacity  in  resisting  ordinary  weathering.  The  vertical 
walls  and  escarpments  formed  by  this  soil  show  one  of  its 
striking  physical  characteristics.  In  China  caves  that  house 
thousands  of  persons  are  dug  in  the  defiles  and  canons  ex- 
isting in  this  deposit.  Another  feature  of  loess  is  the  pres- 
ence, especially  in  the  subsoil,  of  minute  vertical  canals  lined 
with  a  deposit  of  calcium  carbonate.  These  canals  are  sup- 
posed to  give  the  soil  its  vertical  cleavage  and  its  tenacity. 
The  particles  of  loess  are  usually  unweathered  and  angular. 
Quartz  seems  to  predominate,  but  large  quantities  of  feld- 


GEOLOGICAL  CLASSIFICATION  OF  SOILS        63 

spar,   mica,   hornblende,   augite,   calcite   and   other   minerals 
are  found. 

A  few  typical  analyses  are  given  below  which  show  the 
variability  that  may  be  expected,  especially  in  the  nitrogen, 
phosphoric  acid,  potash,  and  lime. 

Table  X 

ANALYSES    OF   AMERICAN   LOESS   SURFACE   SOILS1 


Constituents 

l 

2 

3 

4 

Si02 

Ti02 

A1203 

Fe„03 

MgO 

CaO 

Nao0 

K26 

P205 

N 

71.30 

.60 

11.47 

4.05 

1.10 

1.38 

1.95 

2.40 

.23 

.22 

81.13 

.78 

8.52 

2.92 

.39 

.31 

.52 

1.78 

.08 

.11 

86.96 

.69 

4.69 

2.86 

.43 

.71 

1.07 

.91 

.07 

.11 

69.66 
1.72 

12.71 
4.89 
1.28 
1.09 
1.17 
2.42 
.15 
.23 

Whenever  moisture  relations  are  favorable,  loess  is  an 
exceedingly  fertile  soil.  Under  heavy  cropping,  especially 
when  little  in  the  way  of  organic  or  mineral  matter  is  re- 
turned, this  soil  shows  a  need  of  phosphoric  acid  and  lime, 
the  application  of  which  is  becoming  part  of  good  farm  prac- 
tice in  the  Central  West.     Considering  the  wide  extension  of 

1 1.     Marshall  silt  loam,  Pottawattamie  Co.,  la. 

Bennett,  H.  H.,  Soils  and  Agriculture  of  the  Southern  States, 
p.  332;  New  York,  1921. 

2.  Memphis  silt  loam,  Grenada  Co.,  Miss. 

Eobinson,  W.  O.,  et  al.,  Variation  in  the  Chemical  Composition 
of  Soils;  U.  S.  Dept.  Agr.,  Bui.  551,  June,  1917. 

3.  Cherokee  silt  loam,  Cherokee  Co.,  Kan. 

Bennett,  H.  H.,  Soils  and  Agriculture  of  the  Southern  States, 
p.  332;  New  York,  1921. 

4.  Silt  loam,  Weeping  Water,  Neb. 

Alway,  F.  J.,  and  Rost,  C.  O.,  The  Loess  Soils  of  the  Nebraska 
Portion  of  the  Transition  Begion,  Part  IV;  Soil  Sci.,  Vol.  I, 
No.  5,  p.  431,  May  1916. 


64  NATURE  AND  PROPERTIES  OF  SOILS 

the  loess  and  the  great  variety  of  climate  and  cropping  to 
which  it  is  subject,  it  may  be  classed  as  one  of  the  world's 
most  important  soils.  In  the  United  States  it  is  the  great 
maize-producing  soil  of  the  upper  Mississippi  Valley. 

36.  Other  seolian  soils. — The  term  ' '  adobe ' '  is  applied  to  a 
fine  calcareous  clay  or  silt  formed  in  a  manner  somewhat 
like  loess.  It  is  supposed  that,  while  part  of  the  deposit  came 
from  the  waste  of  talus  slopes  as  mountains  were  weathered 
under  conditions  of  aridity,  the  remainder  had  aeolian  origin. 
Certain  characteristics  also  seem  to  indicate  that  the  valley 
adobe  might  have  been  deposited  almost  entirely  by  water. 
It  appears,  therefore,  that,  while  the  physical  characteristics 
of  all  adobe  are  somewhat  similar,  its  mode  of  origin  and 
chemical  composition  may  be  variable. 

Like  the  loess,  the  adobe  is  an  exceedingly  rich  soil,  but 
it  occurs  in  an  arid  or  a  semi-arid  region.  When  irrigated 
its  fertility  seems  inexhaustible.  It  is  found  in  Colorado, 
Utah,  southern  California,  Arizona,  New  Mexico,  and  Texas. 
It  has  an  especially  wide  distribution  in  New  Mexico.  Like 
loess,  its  elevation  is  variable,  ranging  from  sea  level  in  Cali- 
fornia and  Arizona  to  6000  feet  along  the  eastern  border  of 
the  Rocky  Mountains.  Its  maximum  thickness  cannot  be  esti- 
mated, as  it  is  very  little  eroded  and  is  supposed  to  be  still 
accumulating.  Some  valleys  are  known  to  be  filled  to  a  depth 
of  3000  feet  with  this  material.  Its  characteristics  are  its 
fine  texture,  its  great  depth,  its  wide  distribution,  and  its 
great  fertility  when  moisture  conditions  are  suitable  for  crop 
growth. 

Sand  dunes  are  the  outgrowth  of  two  conditions — a  large 
quantity  of  sand  and  a  wind  that  blows  in  a  more  or  less 
prevailing  direction.  Under  such  conditions  the  sand  and 
other  fine  materials  are  not  only  blown  into  heaps,  but  also 
tend  to  move  in  the  direction  of  the  prevailing  wind.  Sand 
dunes  may  often  assume  gigantic  proportions,  sometimes  be- 
ing several  hundred  feet  high  and  twenty  or  thirty  miles 


GEOLOGICAL  CLASSIFICATION  OF  SOILS        65 


long.  In  such  proportions  they  become  a  grave  menace  to 
agriculture,  not  only  because  they  are  an  absolutely  valueless 
medium  for  plant  growth,  but  also  because  they  cover  fertile 
lands  and  entirely  blot  out  all  vegetation. 

From  early  geologic  times  deposits  of  the  very  fine  material, 
that  is  continually  being  ejected  from  volcanoes,  have  been 
distributed  over  the  earth's  surface.  These  deposits  are 
usually  flour-like,  and  while  at  one  time  they  probably  cov- 
ered many  square  miles  of  territory,  they  have  succumbed 
very  largely  to  erosion  and  denudation,  and  only  remnants 
are  found  at  the  present  time.  Such  material  may  be  found 
in  Montana,  Nebraska,  and  Kansas.  ^Eolian  deposits  of  this 
character  are  usually  rather  porous  and  light,  and  are  likely 
to  be  highly  siliceous.  They  are  not  of  great  agricultural 
importance,  except  in  certain  localities. 

37.  Resume. — The  geological  classification  of  soils  pro- 
vides a  logical  basis  for  the  discussion  of  the  formation,  char- 
acter, and  agricultural  value  of  soils  in  general.  A  detailed 
consideration  on  any  other  basis  would  lead  to  endless  con- 
fusion and  repetition.  In  classifying  soils  a  study  must  be 
made  not  only  of  the  past  effects  but  of  the  present  influences 
of  the  soil-forming  processes,  and  while  the  conclusions  and 
observations  are  apparently  purely  agricultural  in  nature, 
they  really  spring  from  a  geochemical  foundation. 

With  such  a  classification  at  hand  one  cannot  fail  to  under- 
stand the  occurrence  of  so  many  distinct  and  different  types 
of  soil.  It  is  really  difficult  to  see  why  soils  do  not  present 
greater  differences  and  why  transition  types  do  not  utterly 
prevent  clean-cut  field  distinctions.  In  such  soil  study  the 
all-important  character  of  climatic  control  must  always  be 
remembered.  Weathering  is  strictly  a  climatic  influence  and 
crop  adaptation  is  usually  dominated  by  climate  rather  than 
by  soil. 


CHAPTER  IV 

THE   SOIL   PARTICLE   AND    CERTAIN  IMPORTANT 
RELATIONS 

An  examination  of  a  soil,  however  cursory,  immediately 
reveals  that  it  is  made  up  of  irregular  fragments  of  mineral 
material  mixed  and  more  or  less  coated  with  organic  matter. 
These  fragments,  varying  in  size  from  particles  easily  discern- 
ible by  the  naked  eye  to  particles  so  fine  as  to  be  invisible 
under  the  ultra-microscope,  determine  to  a  very  large  degree 
the  complex  relationships  of  soil  to  plant.  The  movement  of 
air  in  the  soil,  the  circulation  of  the  water,  chemical  reactions 
resulting  in  solution,  and  the  presence  and  virility  of  the 
various  organisms  are  determined  largely  by  the  size  of  par- 
ticles making  up  a  soil  and  by  the  proportion  and  condition 
of  the  organic  material  present.  In  expressing  the  size  or 
sizes  of  particles  making  up  a  soil,  the  term  texture  is  used. 
Thus  a  soil  texture  may  be  coarse,  medium,  or  fine,  indicating 
that  the  particles  making  up  the  soil  conform  in  general  to 
such  description. 

Texture  is  a  condition  which  can  be  but  little  modified  in 
a  normal  soil.  We  have  seen  how  a  rock  can  be  disintegrated, 
decomposed  and  gradually  built  into  a  soil.  A  change  in 
texture  has  been  wrought,  but  such  a  process  demands  geo- 
logic ages  for  its  fulfillment.  In  the  time  covered  by  the  life 
of  man,  the  necessary  forces  are  not  active  enough  to  have 
this  effect;  consequently,  as  far  as  the  farmer  is  concerned, 
the  texture  of  the  soil  in  his  field  is  subject  to  but  slight 
alteration.  A  sand  remains  a  sand  and  a  clay  remains  a  clay, 
as  far  as  practical  considerations  are  concerned.     Changes 

66 


THE  SOIL  PARTICLE 


67 


in  texture  may  be  made  on  a  small  scale  by  mixing  two  soils, 
but  this  is  not  practicable  in  the  field. 

38.  Separation  and  classification  of  soil  particles. — In 
order  that  the  particles  of  soil,  varying  so  tremendously  in 
size  as  they  do,  may  be  studied  successfully,  they  must  be 
separated  into  groups  according  to  their  diameters.  The  va- 
rious groups  are  spoken  of  as  soil  separates.  Such  a  grouping 
is  of  course  arbitrary,  and  must  meet  certain  theoretical  as 
well  as  practical  requirements.  It  must  be  simple,  short,  and 
capable  of  expressing  in  a  practical  way  the  physical  char- 
acter of  the  soil.  Moreover,  it  must  lend  itself  to  the  actual 
separation  and  percentage  evaluation  of  each  group.  This 
analytical  procedure  is  called  a  mechanical  analysis. 

With  the  large  number  of  different  methods  of  mechanical 
soil  analyses,  there  has  arisen  considerable  variation  in  tex- 
tural  groupings  expressed  in  diameter  of  particles.  This 
would  naturally  occur  because  of  the  differences  in  degree  of 
refinement,  which  the  various  methods  of  separation  allow, 
and  also  because  of  the  uses  which  the  investigators  wished  to 


Table  XI 

THE  NAMES  AND   RANGES  IN   SIZE   OF  SOIL  PARTICLES  AS   ESTAB- 
LISHED BY  THE  BUREAU  OF  SOILS  CLASSIFICATION1 


Separate 

Size   in 
Millimeters 

Fine  Sandy 
Loam 

Clay 

1.  Fine  Gravel.  .  .. 

2.  Coarse  Sand. . . 

3.  Medium  Sand. . 

4.  Fine  Sand 

5.  Very  Fine  Sand 

6.  Silt 

m.m. 
2—1 
1— .5 
.5— .25 
.25— .10 
.10— .05 
.05— .005 
.005  and  below 

% 

1 

2 

3 

22 

35 

27 

10 

% 
1 
2 
2 
6 
7 

39 

7.  Clay 

43 

1  Briggs,  L.  J.,  et  al.,  The  Centrifugal  Method  of  Soil  Analysis;  U.  S. 
Dept.  Agr.,  Bur.  Soils,  Bui.  24,  1904. 


68 


NATURE  AND  PROPERTIES  OF  SOILS 


make  of  such  analyses.  The  grouping  established  by  the 
United  States  Bureau  of  Soils  is  met  with  in  all  soil  literature 
and  is  really  the  standard  classification  for  this  country. 
Table  XI  sets  forth  the  essential  points  of  the  Bureau  of  Soils 
classification.  In  the  first  column  are  given  the  names  of  the 
various  separates,  and  in  the  second  the  range  in  size  of  each 
group.  Columns  three  and  four  show  the  percentages  of  each 
separate  in  two  very  different  specimen  soils,  a  sandy  loam 
and  a  clay.  In  order  to  obtain  such  figures,  a  sample  of  the 
dry  soil  must  actually  be  separated  into  the  arbitrary  groups 
and  the  percentage  of  each  group  to  the  whole  soil  calculated 
from  the  dry  weights  obtained.  This  operation  is  the  mechan- 
ical analysis  already  mentioned. 

This  classification  establishes  seven  distinct  groups1  rang- 

1  Various    Textural    Classifications    Other    Than    That    of    the 
Bureau  of  Soils  Used  in  the  Mechanical  Analyses 
of  Soils.    Expressed  in  Diameter  of  Par- 
ticles in  Millimeters 


Separate 

Osborne1 

HlLGARD3 

English3 

Atterberg4 

1 
2 
3 
4 
5 
6 

3.00-1.00 
1.00-  .50 
.50-  .25 
.25-  .05 
.05-  .01 
.01 

3.00-1.00 
1.00-  .50 
.50-  .30 
.30-  .16 
.16-  .12 
.12-  .07 
and  six 
other  divisions 

1.00  -  .200 
.20  -  .040 
.04  -  .010 
.01  -  .002 
.002 

20.00  -2.00 

2.00  -  .20 

.20  -  .02 

.02  -  .002 

.002 

1  Osborne,  T.  B.,  Methods  of  Mechanical  Soil  Analysis;  Ann.  Rep. 
Conn.  Agr.  Exp.  Sta.,  1886,  pp.  141-158;  1887,  pp.  144-162;  1888,  pp. 
154-157. 

'Hilgard,  E.  W.,  Methods  of  Physical  and  Chemical  Soil  Analysis; 
Ann.  Rep.  Cal.  Agr.  Exp.  Sta.,  1891-1892,  pp.  241-257. 

3  Hall,  A.  D.  and  Russell,  E.  J.,  Soil  Surveys  and  Soil  Analysis;  Jour. 
Agr.  Science,  Vol.  IV,  part  2,  pp.  182-223,  1911. 

4  Atterberg,  A.,  Die  Mechanische  Bodenanalyse  und  die  KlassifiTcation 
der  Mineralboden  Schwedens.  Internat.  Mitt.  f.  Bodenkunde,  Band  II, 
Heft  4,  Seite  312-342,  1912.  Schucht,  F.,  fiber  die  Sitzung  der  Inter- 
national Konvmission  fur  die  Mechanische  und  Physikalische  Boden- 
untersuchung  in  Berlin  am  31,  October  1913;  Internat.  Mitt.  f.  Boden- 
kunde, Band  IV,  Heft  I,  Seite  1-31,  1914. 


THE  SOIL  PARTICLE  69 

ing  from  fine  gravel;  readily  visible  to  the  naked  eye,  to  the 
clay  separate,  the  largest  particle  of  which  is  .005  of  a  milli- 
meter or  .0002  of  an  inch  in  diameter.  The  stone  and  large 
gravel,  while  they  figure  in  a  practical  examination  and  de- 
scription of  a  soil  in  the  field,  obviously  need  not  be  considered 
in  such  a  classification  as  this. 

The  seven  separates  may  be  thrown  into  two  groups  for 
a  preliminary  examination  on  the  basis  of  visibility  to  the 
naked  eye.  The  gravel  and  sand  particles  are  readily  seen, 
while  the  silt  and  especially  the  clay  particles  are  invisible 
as  individuals,  although  some  of  the  larger  silt  particles  may 
be  seen  with  the  naked  eye  when  suspended  in  water.  The 
gravel  and  sand,  when  dominant  in  a  soil,  give  properties 
known  to  every  one  as  sandy,  while  if  the  soil  is  made  up 
largely  of  silt  and  clay,  its  plasticity  and  stickiness  proclaim  it 
as  clayey  in  nature.  The  characteristics  of  the  two  soils  of  the 
above  table  may  be  read  easily  from  their  mechanical  analyses. 
The  classification,  therefore,  meets  the  criteria  already  estab- 
lished. It  is  simple,  easy  to  remember,  and  is  capable  of 
expressing,  to  a  certain  extent  at  least,  the  dominant  physical 
characters  of  soils.  As  will  be  shown  below,  it  lends  itself 
to  the  quantitative  separation  of  a  soil,  the  so-called  mechan- 
ical analysis. 

39.  The  beaker  method  of  mechanical  analysis. — When 
fragments  of  rock  or  soil  are  suspended  in  water  they  tend 
to  sink  slowly,  and  it  is  a  well  recognized  fact  that,  other 
things  being  equal,  the  rate  of  settling  depends  on  the  size 
of  the  particle.  As  the  particle  is  decreased  in  size  its  weight 
decreases  faster  than  the  surface  exposed  to  the  buoyant  force 
of  the  water.  As  a  consequence  the  rapidity  with  which  the 
soil  particles  settle  is  more  or  less  proportional  to  their  size. 
The  suspension  of  a  sample  of  soil  would,  therefore,  be  the 
first  step  in  mechanical  separation  by  water;  the  second  step 
would  be  subsidence  and  the  withdrawal  of  each  successive 
grade  of  particles  as  it  slowly  settled;  the  third  step  would 


70  NATURE  AND  PROPERTIES  OF  SOILS 

be  the  determination  of  the  percentage  of  each  grade,  or  group, 
of  particles  as  based  on  the  original  sample.  This  is  precisely 
what  every  method  of  mechanical  analysis  in  which  water  is 
utilized  aims  to  do,  although  the  irregularity  in  the  shape  of 
the  particles  prevents  to  a  certain  extent  a  perfect  separa- 
tion. The  apparatus  and  technique  of  the  various  methods 
employed  are  generally  rather  complicated. 

One  of  the  earliest  and  most  useful  methods  to  be  perfected 
was  the  separation  of  the  various  grades  of  soil  by  simple 
subsidence  in  a  column  of  still  water.  This  is  commonly 
spoken  of  as  the  Osborne  beaker  method.1  The  determination 
is  very  simple.  The  soil  sample  is  first  fully  deflocculated  and 
thrown  into  suspension,  each  particle  functioning  separately. 
Beakers  are  commonly  used  as  containers,  but  any  vessel  that 
is  relatively  deep  will  do  for  the  determination.  The  larger 
particles,  gravel  and  sand,  will  of  course  settle  first,  and  the 
finer  silts  and  clays  may  be  decanted  off.  As  the  sands  carry 
finer  particles  down  with  them,  the  suspension  and  subsidence 
must  be  repeated  a  number  of  times.  The  sands  are  later 
dried  and  sieved  into  their  respective  groups.  The  silt  and 
clay  particles,  thus  decanted,  may  be  separated  from  each 
other  by  subsidence  as  above  described.  The  time  necessary 
for  such  decantation  as  will  leave  in  suspension  only  particles 
below  a  given  size  is  determined  by  the  examination  of  a  drop 
of  the  suspension  under  a  microscope  fitted  with  an  eyepiece 
micrometer.  In  this  way  the  size  of  the  particles  decanted 
may  be  measured  accurately.     (See  Fig.  14.) 

The  four  steps  in  this  method  of  separation  are :  defloccula- 
tion  of  the  sample;  separation  by  successive  subsidence  and 
decantation ;  evaporation  to  dryness  of  the  separates  and  the 
sieving  of  the  sands;  and  the  weighing  of  the  separates  and 
the  calculation  of  percentages  based  on  the  original  dry  sam- 

1  Osborne,  T.  B.,  Methods  of  Mechanical  Soil  Analysis;  Ann.  Rep. 
Conn.  Agr.  Exp.  Sta.,  1886,  pp.  141-158;  1887,  pp.  144-162;  1888,  pp. 
154-157. 


THE  SOIL  PARTICLE 


71 


pie.  The  method,  however,  is  slow,  as  the  time  necessary  for 
each  subsidence  of  the  finer  particles  is  very  great  and  the 
number  of  individual  subsidences  is  large.  As  a  consequence, 
it  has  been  superseded  by  methods  that  utilize  centrifugal 
force  for  the  finer  separations,  while  retaining  gravity  for 
removing  the  various  grades  of  sand. 


•   :  I  '  *    "  '  •      ■    •  r  "•    *  '  i  .*  * 


• 


Fig.  14. — Diagram  showing  the  relative  sizes  of  soil  particles  as  they 
appear  under  a  microscope  with  eye-piece  micrometer.  Particles 
one  space  or  less  in  diameter  are  clay;  from  one  space  to  ten,  silt 
and  above  ten  spaces,  very  fine  sand. 


40.  Bureau  of  Soils  centrifugal  analysis. — Of  the  cen- 
trifugal methods  used  in  mechanical  analysis  that  employed 
by  the  United  States  Bureau  of  Soils1  is  the  most  successful. 
A  five-gram  sample  of  well-pulverized  soil  is  put  into  a  shaker 
bottle  of  about  250  cubic  centimeters  capacity.  This  bottle 
is  filled  about  two-thirds  full  of  water  so  that  in  shaking  the 
disintegrating  force  of  the  liquid  may  be  utilized.     A  few 

1  Fletcher,  C.  C.  and  Bryan,  H.,  Modifications  of  the  Method  of  Soil 
Analysis;  U.  S.  Dept.  Agr.,  Bur.  Soils,  Bui.  84,  1912. 


72  NATURE  AND  PROPERTIES  OF  SOILS 

drops  of  ammonia  are  added  to  dissolve  the  organic  matter 
and  to  make  deflocculation  easier.  The  sample  is  then  agi- 
tated in  the  bottle  until  disintegration  is  complete.  This 
period  ranges  from  five  to  twenty  hours,  depending  on  the 
sample.     (See  Fig.  15.) 

The  separation  of  the  silt  and  the  clay  from  the  sands  is 
made  in  the  shaker  bottle  by  simple  subsidence,  the  time  for 
decantation  being  determined  by  a  microscopic  examination 
of  a  drop  of  the  suspension.  The  silt  and  the  clay  are  de- 
canted directly  into  a  test-tube  fitted  into  a  centrifuge.  Whirl- 
ing at  the  rate  of  800  to  1000  revolutions  a  minute  will  cause 
the  subsidence  of  the  silt  to  the  bottom  of  the  test-tube  in  a 
few  minutes.  The  clay  is  then  decanted.  The  microscope  is 
necessary  here  in  order  to  determine  when  the  settling  of  the 
silt  is  complete.  As  small  particles  tend  to  cling  to  the  larger 
particles  the  entire  operation  must  be  repeated  several  times; 
therefore  the  processes  of  gravity  subsidence  and  centrifugal 
subsidence  are  carried  on  side  by  side,  material  being  con- 
stantly poured  from  the  shaker  bottle  into  the  centrifuge  tubes 
and  from  the  test-tubes  into  the  receptacles  for  the  clay. 

The  centrifuge  is  usually  large  enough  to  allow  the  separa- 
tion of  several  duplicate  samples  at  once.  The  various  sep- 
arates made  by  this  method  are  dried  and  weighed.  The 
sands,  which  are  obtained  in  bulk,  are  further  separated  by 
sieves  into  the  grades  desired.  When  a  large  quantity  of 
organic  matter  is  present  it  must  be  determined  and  included 
in  the  final  report  on  the  sample. 

This  method  of  mechanical  analysis  as  perfected  by  the 
Bureau  of  Soils  has  been  very  commonly  adopted  by  soil  work- 
ers. It  has  many  advantages  over  other  methods.1  In  the 
first  place,  it  is  rapid,  often  requiring  only  hours  where  other 

1  Classification  of  the  Various  Methods  of  Mechanical  Analysis : 

f  Wet 
i       ev*M,«     J    nr.  Used  to  separate  sands  in  practically  all 

<  j£  methods. 


THE  SOIL  PARTICLE 


73 


methods  take  days  for  completion ;  secondly,  it  is  simple,  and 
the  technique  of  the  separation  is  easily  acquired;  thirdly,  in 
the  decantations  no  very  large  amount  of  water  is  accumulated 
with  the  separates,  except  for  the  clay,  and  thus  the  time  and 
cost  of  evaporation  is  reduced.  The  clay,  moreover,  may  be 
as  accurately  determined  by  difference  as  by  direct  methods, 
thus  allowing  a  further  saving  of  time.  While  the  method 
is  accurate  only  within  one  per  cent.,  it  is  sufficiently  precise 
for  all  practical  purposes. 

41.  Physical  characters  of  the  soil  separates. — It  is  im- 
mediately apparent  that  as  these  groups  vary  in  size  they 
must  exhibit  properties,  especially  physical  ones,  which  are 
widely  different.  These  properties  should  in  turn  be  imparted 
to  the  soil  of  which  the  separates  form  a  part.    If  a  person  is 


2.     Air 


(Cushman's1  air  elutriator). 


3.     Water 


I 


Gravity     (Schone's2    elutriator 
and    Hilgard's*    churn    elutria- 

In  motion  -J  tor). 

Centrifugal  ( Yoder  's  *  Centrifu- 
gal elutriator) . 
"Gravity        (Osborne's       beaker 
method  and  Atterberg's5  modi- 

At  rest       <  fied  silt  cylinder). 

Centrifugal    (Bureau    of    Soils 
method) . 

For  a  detailed  discussion  of  all  methods  of  mechanical  analysis  see 
Wiley,  H.  W.,  Agricultural  Analysis,  Vol.  I,  pp.  195-276;  Easton,  Pa., 
1906. 

1Cushman,  A.  S.  and  Hubbard,  P.,  Air  Elutriation  of  Fine  Powders; 
Jour.  Amer.  Chem.  Soc,  Vol.  29,  No.  4,  pp.  589-597,  1907. 

aSchone,  E.,  trber  Schlammansalyse;  Bui.  Soc.  Imperiale  des  Natural- 
istes  de  Moscow,  40,  Part  1,  p.  324,  1867.  trber  Schlammanalyse  und 
einen  neuen  Schlanvmapparat ;  Berlin,  1867.  Also  see  Wiley,  H.  W., 
Agricultural  Analysis,  Vol.  I,  pp.  231-241;  Easton,  Pa.,  1906. 

'Hilgard,  E.  W.,  Methods  of  Physical  and  Chemical  Soil  Analysis; 
Ann.  Eep.  Calif.  Agr.  Exp.  Sta.,  pp.  241-257,  1891-1892. 

4  Yoder,  P.  A.,  A  N*ew  Centrifugal  Soil  Elutriator;  Utah  Agr.  Exp. 
Sta.,  Bui.  89,  1904. 

6  Appiani,  G.,  trber  einen  Schlammapparat  fur  die  Analyse  der  Boden- 
und  Thonarten;  Forsch.  a.d.  Gebiete  d.  Agri-Physik,  Band  17,  Seite  291- 
297,  1894.  Atterberg,  A.,  Die  Mechanische  Bodenanalyse  und  die  Klassi- 
fikation  der  Mineralboden  Schwedens;  Internat.  Mitt.  f.  Bodenkunde, 
Band  II,  Heft  4,  Seite  312-342,  1912. 


74 


NATURE  AND  PROPERTIES  OP  SOILS 


conversant  with  these  various  values,  a  mechanical  analysis 
should  reveal  at  a  glance  certain  soil  conditions,  which  may 
or  may  not  be  conductive  to  the  best  plant  growth. 

The  sands  and  the  gravel,  because  of  their  sizes,  function 
as  separate  particles.  They  are  irregular  and  rounded,  the 
continual  rubbing  that  they  have  received  being  sufficient  in 

many  cases  to  have  ef- 
faced their  angular  char- 
acter. They  exhibit  very 
low  plasticity  and  cohe- 
sion, and  as  a  consequence 
are  little  influenced  by 
changes  in  water  content. 
Their  water-holding  ca- 
pacity is  low,  and  because 
of  the  large  size  of  the 
spaces  between  each  sep- 
arate particle  the  passage 
of  percolating  water  is 
rapid.  They,  therefore, 
facilitate  drainage  and 
encourage  good  air  move- 
ment. In  all  the  grades 
of  sand,  the  separate  par- 
ticles are  visible   to   the 


naked    eye,    a    condition 


Fig.  15.  —  Apparatus  for  making  a 
mechanical  analysis  of  soils. 
Shaker-bottle  (A),  shaking-rack  (B), 
sieves  (C),  centrifuge  (D)  and  cen- 
trifuge-tube   (E). 

impossible  with  the  silt 
and  clay  groups.  Soil  containing  much  sand  or  gravel,  there- 
fore, is  of  open  character,  possessing  good  drainage  and 
aeVation,  and  is  usually  in  a  loose  friable  condition. 

The  clay  and  silt  particles  are  very  minute,  many  of  the 
former  being  so  small  as  to  be  invisible  under  the  ultra-micro- 
scope. Both  groups  are  really  shreds  and  fragments  of  min- 
erals often  rather  gelatinous  in  nature.  The  clay  particles 
are  highly  plastic  and  when  kneaded  with  just  the  correct 


THE  SOIL  PARTICLE  75 

amount  of  water  they  become  sticky  and  impervious.  On  dry- 
ing, they  shrink  with  the  absorption  of  considerable  heat.  On 
wetting,  again  swelling  occurs.  The  absorptive  capacity  of 
clay  material  for  water,  gases,  and  soluble  salts  is  very  high, 
due  to  the  presence  of  colloidal  material.1  As  material  in  a 
colloidal  condition  is  very  finely  divided,  it  is  found  largely 
in  the  heavier  types  of  soil.  Some  clays  carry  very  large 
amounts  of  material  in  a  colloidal  state.  Silt  possesses  the 
same  properties  of  plasticity,  cohesion,  and  absorption  as  does 
clay,  but  to  a  less  extent,  because  the  particles  of  the  former 
are  larger  than  those  of  the  latter.  The  presence  of  silt  and 
especially  clay  in  soil  imparts  to  it  a  heavy  texture,  with  a 
tendency  to  slow  water  and  air  movement.  Such  a  soil  is 
highly  plastic,  but  becomes  sticky  when  too  wet,  and  hard 
and  cloddy  when  too  dry.  The  expansion  and  the  contraction 
on  wetting  and  drying  are  very  great.  The  water-holding 
capacity  of  a  clayey  or  silty  soil  is  high.  Such  soils  are  spoken 
of  as  heavy  because  of  their  working  qualities  in  the  field  in 
contrast  to  the  easily  tilled  sandy  soils. 

42.  The  mineralogical  and  chemical  characteristics  of 
soil  separates. — From  the  mineralogical  standpoint  there 
are  often  considerable  differences  between  the  soil  separates, 
especially  when  the  sands  and  clays  are  compared.  Quartz 
would  naturally  be  expected  to  persist  and  because  of  its  low 
solubility  would  very  soon  be  dominant  not  only  in  the  coarser 
separates  but  in  the  silt  and  clay  as  well.  Other  minerals, 
such  as  the  feldspars,  hornblende,  mica,  and  augite  being  less 
resistant  would  concentrate  in  the  finer  separates.    This  tend- 

1  The  colloidal  state — when  material  is  in  a  very  fine  state  of  division, 
approaching  but  not  attaining  a  molecular  condition  (true  solution),  it 
assumes  certain  characteristic  properties,  such  as  high  absorption  for 
water,  gases,  and  salts  in  .solution.  It  may  also,  under  certain  conditions, 
cause  a  marked  increase  in  plasticity  and  cohesion.  The  colloidal  con- 
dition is  purely  physical  and  depends  -on  fineness  of  division,  the 
particles  being  molecular  complexes.  Material  in  a  colloid  state  is 
heterogeneous  and  is  dispersed  through  a  second  material  called  the 
dispersive  medium. 


76 


NATURE  AND  PROPERTIES  OF  SOILS 


ency  together  with  the  formation,  as  weathering  proceeds, 
of  the  fine  coloidal-like  epidote,  chlorite  and  similar  groups, 
should  in  general  keep  the  percentage  of  minerals  other  than 
quartz  higher  in  the  finer  portions  of  a  soil.1  The  following 
data  sustain  this  assumption: 

Table  XII 

MINERALS    OTHER   THAN    QUARTZ    IN    THE    SANDS   AND    SILTS    OF 
VARIOUS  SOILS  2 


Soils 

Minerals  Other  Than  Quartz  in 

Sands 

Silts 

12  Residual   . 

15% 
12 
5 
37 

21% 
15 

18 

6  Glacial  and 
4  Marine  .... 

Loessial 

3  Arid 

42 

It  is  to  be  seen  immediately  that  in  every  case  the  silt  car- 
ries a  large  quantity  of  the  important  soil-forming  minerals 
and  a  smaller  amount  of  quartz  than  does  the  sand.  This  re- 
veals at  least  one  of  the  reasons  for  the  greater  fertility  and 
lasting  qualities  of  fine-textured  soils  as  far  as  agricultural 
operations  are  concerned.    It  is  important  to  note,  however, 

1  A  petrographic  analysis  as  now  developed  is  very  unsatisfactory  as  it 
throws  practically  no  light  on  the  character  of  the  clay  group  because 
of  the  extreme  fineness  of  this  material.  Even  for  silt  the  results  are 
unsatisfactory  and  difficult  to  express  quantitatively.  The  correlation 
of  a  petrographic  analysis  and  productivity  is  vague. 

aMcCaughey,  W.  G.,  and  Fry,  W.  H.,  The  Microscopic  Determina- 
tion of  Soil-forming  Minerals;  U.  S.  Dept.  Agr.,  Bur.  of  Soils,  Bui.  91, 
1913.  See  also,  Plummer,  J.  K.,  Eelation  of  the  Mineralogical  and 
Chemical  Composition  to  the  Fertility  Requirements  of  North  Carolina 
Soils;  N.  C.  Agr.  Exp.  Sta.,  Tech.  Bui.  9,  1914.  Plummer,  J.  K., 
Petrography  of  Some  North  Carolina  Soils  and  its  Relationship  to  their 
Fertilizer  Requirements;  Jour.  Agr.  Res.,  Vol.  V,  No.  13,  pp.  569-581, 
1915.  Robinson,  W.  O.,  The  Inorganic  Composition  of  Some  Important 
American  Soils;  U.  S.  Dept.  Agr.,  Bui.  122,  Aug.,  1914.  Shorey,  F.  C, 
et  ah,  Calcium  Compounds  in  Soils;  Jour.  Agr.  Res.,  Vol.  VII,  .No.  3, 
pp.  57-77.     Jan.,  1917. 


THE  SOIL  PARTICLE  77 


that,  although  quartz  is  the  predominating  mineral  in  sands, 
all  the  common  soil-forming  minerals  are  usually  accessory.1 

It  is  interesting  in  passing  to  observe  the  differences  ex- 
hibited by  the  various  soil  provinces  although  the  number  of 
samples  shown  by  Table  XII  are  far  too  small  for  definite 
conclusions.  The  marine  soils  are  particularly  low  compared 
with  the  residual  and  glacial,  due  to  the  hard  usage  which 
the  soil  material  of  the  former  has  received.  No  significant 
differences  exist  between  the  glacial  and  residual  soils.  The 
arid  soils,  however,  are  markedly  higher  in  the  important  min- 
erals due  to  the  suppression  of  chemical  weathering  and  the 
activity  of  the  physical  agents.  The  silica  in  such  soils  is 
held  as  complex  silicates,  which  carry  the  elements  that  are 
so  important  in  plant  development.  Although  these  data  are 
based  on  but  a  few  samples,  they  are  so  concordant  with  what 
would  naturally  be  expected  that  these  general  conclusions 
cannot  be  avoided. 

The  mineralogical  examination  has  revealed  a  larger  per- 
centage of  such  minerals  as  feldspars,  mica,  hornblende,  and 
the  like,  in  the  finer  separates.  A  larger  percentage  of  the 
important  nutrient  elements  would,  therefore,  be  expected  in 
those  groups.  The  following  data,2  compiled  from  work  per- 
formed by  the  United  States  Bureau  of  Soils,  substantiate  this 
assumption.     (See  Table  XIII,  page  78.) 

It  is  evident  that  the  finer  portions  of  soil  are  in  general 

1  Below  is  given  the  mineralogical  description  of  a  loessial  silt  loam  of 
the  Marshall  Series  from  Missouri:  Kobinson,  W.  O.,  The  Inorganic 
Composition  of  Some  Important  American  Soils;  U.  S.  Dept.  Agr.,  Bui. 
122,  Aug.,  1914. 

Very  fine  sand — minerals  other  than  quartz,  20  per  cent.  Orthoclase, 
10  per  cent.  Muscovite,  2  per  cent.  Biotite,  magnetite,  epidote,  albite, 
labradorite,  oligoclase,  tourmaline,  zircon,  garnet,  and  augite  are  also 
present. 

Silt — Minerals  other  than  quartz,  34  per  cent.  Orthoclase,  4  per  cent. 
Muscovite,  4  per  cent.  Biotite,  magnetite,  epidote,  albite,  labradorite, 
oligoclase,  tourmaline,  rutile,  glaucophane,  hornblende,  and  augite  are 
also  present. 

2  Failyer,  G.  H.,  and  Others,  The  Mineral  Composition  of  Soil  Particle^ 
U.  S.  Dept.  Agr.,  Bur.  Soils,  Bui.  54,  1908. 


78 


NATURE  AND  PROPERTIES  OF  SOILS 


Table  XIII 

CHEMICAL  COMPOSITION   OF  VARIOUS  SOIL  SEPARATES 


Num- 

Percentage of 

Percentage  of 

Percentage  of 

Soils 

ber  of 
Sam- 

P30, IN 

K2Qin 

CaOiN 

ples. 

Sand 

Silt 

Clay 

Sand 

Silt 

Clay 

Sand 

Silt 

Clay 

Crystalline 

Residual  . . . 

3 

.07 

.22 

.70 

1.60 

2.37 

2.86 

.50 

.82 

.94 

Limestone 

Residual  . . . 

3 

.28 

.23 

.37 

1.46 

1.83 

2.62 

12.26 

10.96 

9.92 

Coastal  Plain. 

7 

.03 

.10 

.34 

.37 

1.33 

1.62 

.07 

.19 

.55 

Glacial  and 

Loessial  . . . 

10 

.15 

.23 

.86 

1.72 

2.30 

3.07 

1.28 

1.30 

2.69 

Arid 

2 

.19 

.24 

.45 

3.05 

4.15 

5.06 

4.09 

9.22 

8.03 

richer  in  phosphoric  acid,  potash  and  lime  than  the  coarser. 
As  would  be  expected  the  sands,  silts,  and  clays  of  arid  soils 
show  less  difference  than  those  of  the  other  provinces.  Under 
arid  conditions  the  sands  have  not  as  yet  become  depleted  of 
their  store  of  essential  elements.  Average  figures  compiled 
from  Hall 's  analyses  x  of  soils  from  southeastern  England  cor- 
roborate the  data  already  noted.  In  addition,  Hall  shows  that 
the  magnesia,  iron,  and  alumina  are  higher  in  the  finer  sep- 
arates while  there  is  considerably  more  silica  in  the  sand 
groups.2 

1Hall,  A.  D.,  and  Russell,  E.  J.,  Soil  Surveys  and  Soil  Analyses; 
Jour.  Agr.  Sci.,  Vol.  IV,  Part  2,  p.  199,  1911.  Also  A  Report  of  the 
Agriculture  and  Soils  of  Kent,  Surrey,  and  Sussex;  Board  of  Agriculture 
and  Fisheries,  1911.  See  also:  Loughridge,  R.  H.,  On  the  Distribution 
of  Soil  Ingredients  among  Sediments  Obtained  in  Silt  Analyses;  Amer. 
Jour.  Sci.,  Vol.  VII,  p.  17,  1874.  Puchner,  H.,  fiber  die  Vertielung  von 
Nahrstoffen  in  den  Verschieden  Feinen  Bestandteilen  des  Boden;  Landw. 
Ver.  Stat.,  Band  66,  Seite  463-470,  1907.  Hendrick,  J.,  and  Ogg,  W.  J., 
Studies  of  Scottish  Drift  Soil,  Part  I.  The  Composition  of  the  Soil  and 
of  the  Mineral  Particles  Which  Compose  It;  Jour.  Agr.  Sci.,  Vol.  VII, 
Part  4,  pp.  458-469,  Apr.  1916.  McGeorge,  W.  T.,  Composition  of  Ha- 
waiian Soil  Particles;  Haw.  Agr.  Exp.  Sta.,  U.  S.  Dept.  Agr.,  Bui.  42, 
Jan.,  1917.  Robinson,  G.  W.,  Studies  of  the  Pakezoic  Soils  of  North 
Wales;  Jour.  Agr.  Res.,  Vol.  VIII,  Part  3,  pp.  380-381,  June,  1917. 

a  McGeorge 's  investigation  of  the  residual  volcanic  soils  of  Hawaii 
shows  some  noteworthy  exceptions  to  the  work  of  Pailyer  and  Hall  in 


THE  SOIL  PARTICLE 


79 


Table  XIV 

COMPOSITION  OF   SOIL   SEPARATES    (HALL) 


Separate 

Si02 

A1203 

Fea03 

CaO 

MgO 

K20 

P,05 

Coarse  Sand  (1 — .2  mm.) 

93.9 

1.6 

1.2 

.4 

.5 

.8 

.05 

Pine  Sand  (.2— .04  mm.) 

94.0 

2.0 

1.2 

.5 

.1 

1.5 

.1 

Silt  (.04— .01  mm.) 

89.4 

5.1 

1.5 

.8 

.3 

2.3 

.1 

Pine  Silt  (.01— .002  mm.) 

74.2 

13.2 

5.1 

1.6 

.3 

4.2 

.2 

Clay  (Below  .002  mm.) 

53.2 

21.5 

13.2 

1.6 

1.0 

4.9 

.4 

43.  Value  of  a  mechanical  analysis. — It  is  evident  that  a 
proper  interpretation  of  a  mechanical  analysis  will  throw  con- 
siderable light  on  the  probable  condition  of  a  soil,  especially 
physically.  To  the  trained  observer  the  preponderance  of 
sand,  clay,  or  silt  signifies  the  probable  presence  of  certain 
physical  properties,  which  may  affect  the  plant  not  only  me- 
chanically but  physiologically  as  well,  through  air,  water,  and 
nutrient  movement. 

The  chemical  and  mineralogical  phases  of  such  interpreta- 
tion are  also  worthy  of  consideration,  as  the  proportion  of  the 
various  separates  determines  whether  the  essential  nutrient 
will  be  present  in  sufficient  quantities  to  permit  normal  crop 
growth.  Thus  a  mechanical  analysis  not  only  enlightens  as 
to  the  general  properties  of  a  given  soil,  but  when  correlated 
with  other  factors  is  to  some  extent  a  criterion  of  agricultural 
value  and  crop  adaptation.  Some  authors  maintain  that  in 
the  investigation  of  any  soil  a  mechanical  analysis  should  first 
be  made,  as  it  throws  much  light  on  many  properties  of  a  soil. 

44.  Soil  class — how  soils  are  named. — As  a  soil  is  not 
composed  of  particles  of  uniform  size  and  shape,  a  blanket 
term  is  needed,  which  will  not  only  give  some  idea  of  the 
textural  character  of  the  mixture,  for  every  soil  is  a  mixture, 

that  he  found  the  lime  and  magnesia  higher  in  the  coarser  particles  and 
the  silica  higher  in  the  finer  separates.  McGeorge,  W.  T.,  Composition 
of  Eawaiicm  Soil  Particles;  Haw.  Agr.  Exp.  £>ta.,  Bui.  42,  Jan.  1917.' 


80 


NATURE  AND  PROPERTIES  OF  SOILS 


but  at  the  same  time  will  name  it  in  such  a  manner  as  to  reveal 
its  general  physical  peculiarities  and  proportions.  For  this 
class  names,  such  as  sandy  loam,  loam,  silt  loam,  and  the 
like,  are  used.  Class  differs  from  texture,  however,  in  that  it 
has  reference  to  the  properties  exhibited  by  a  soil  rather  than 

_G  RAVEL        5ANb  LOAN  CLAY 


f  e>-  0 


//  SSSSS/s 


/  Ss 


Fig.  16. — Diagram  showing  in  a  general  way  the  mechanical  compo- 
sition of  gravel,  sand,  loam  and  clay  soils  and  indicating  in  addi- 
tion how  some  of  the  more  common  field  names  arise. 


to  any  absolute  grain  size.  Consequently,  there  may  be  a 
number  of  class  names  depending  on  the  proportionate  mix- 
tures of  different  sized  particles  that  occur  in  the  field. 

Class  names  have  originated  through  long  centuries  of  agri- 
cultural observations,  but  of  late  they  have  been  more  or  less 
standardized  because  of  the  necessity  of  a  definite  nomen- 
clature.   In  general,  the  names  used  for  the  soil  classes  are  the 


THE  SOIL  PARTICLE 


81 


same  as  those  employed  in  mechanical  analyses  to  designate 
the  soil  separates.  This  is  rather  unfortunate,  but  it  obviates 
the  increase  of  technical  terms  and  a  little  care  will  prevent 
confusion  in  this  regard.  Four  fundamental  groups  of  soil 
are  recognized:  gravel,  sand,  loam,  and  clay  (See  Fig.  16). 
Gravel  is  a  soil  constituent  that  does  not  often  occur  alone 
and  is  not  of  great  importance  agriculturally  because  of  its 
low  fertility.  The  other  three,  however,  either  alone  or  in 
combination  make  up  most  of  the  arable  soil.  Their  average 
mechanical  analyses  are  set  forth  in  Table  XV. 

Table  XV 

MECHANICAL   ANALYSES   OF    SANDY,    LOAMY   AND    CLAYEY   SOILS1 


Separates 

Sandy 
Soil 

Loamy 

Soil 

Clayey 
Soil 

1.  Fine  Gravel  

2.  Coarse  Sand 

3.  Medium   Sand 

4.  Fine  Sand 

% 

2 

15 

23 

37 

11 

7 

5 

% 

2 
5 
5 
15 
17 
40 
16 

% 
1 
3 
2 

8 

5.  Very  Fine  Sand.  .  .  . 

6.  Silt    

8 
36 

7.  Clay 

42 

The  sand  group  includes  all  soils  of  which  the  silt  and  clay 
separates  make  up  less  than  20  per  cent  of  the  material  by 
weight.  Its  properties  are,  therefore,  characteristically  sandy 
in  contrast  to  the  more  open  character  of  gravel  and  the 
stickier  and  more  clayey  nature  of  the  heavier  groups  of  soil. 
A  soil  to  be  clay  must  carry  at  least  30  per  cent,  of  the  clay 
separate.  It  may  even  have  more  silt  than  clay  but,  since 
the  silt  particles  impart  clayey  characters,  as  long  as  the  per- 
centage of  clay  is  30  or  above,  the  class  name  must  remain 
clay. 

1  Whitney,  M.,  The  Use  of  Soils  East  of  the  Great  Plains  Begion; 
U.  S.  Dept.  Agr.,  Bur.  Soils,  Bui.  78,  p.  12,  1911. 


82  NATURE  AND  PROPERTIES  OP  SOILS 

The  loam  class  is  rather  difficult  to  explain.  In  mechan- 
ical composition  it  is  more  or  less  midway  between  sand  and 
clay.  A  loam  may  be  defined  as  such  a  mixture  of  sand,  silt, 
and  clay  particles  as  to  exhibit  sandy  and  clayey  properties 
in  about  equal  proportions.  It  is  a  half  and  half  mixture 
on  the  basis  of  properties,  although  the  sum  of  the  sands  and 
the  sum  of  the  silt  and  clay  are  generally  near  50  per  cent., 
respectively.  (See  Fig.  16.)  Because  of  the  marked  inter- 
mixture of  coarse,  medium,  and  fine  particles,  loams  are 
usually  soils  of  good  physical  character.  They  generally  pos- 
sess the  desirable  qualities  both  of  sand  and  clay  without 
exhibiting  those  undesirable  properties,  such  as  extreme  loose- 
ness and  low  water  capacity  on  the  one  hand  and  stickiness, 
compactness,  and  slow  air  and  water  drainage  on  the  other. 
Most  of  the  better  soils  are  some  type  of  loam. 

It  is  obvious  that  in  the  field  not  only  various  kinds  of 
gravelly,  sandj',  loamy,  and  clayey  soils  must  occur,  but  the 
groups  must  grade  into  each  other,  thus  giving  rise  to  a  con- 
siderable number  of  field  names.  (See  Pig.  16.)  These  field 
names  are  listed  below : 

Common  Class  Names 

1.  Gravel  9.  Very  fine  sandy  loam 

2.  Coarse  sand  10.  Loam 

3.  Medium  sand.  11.  Silt  loam 

4.  Fine  sand  12.  Silty  clay  loam 

5.  Very  fine  sana  13.  Clay  loam 

6.  Coarse  sandy  loam  14.  Clay 

7.  Sandy  loam  15.  Heavy  clay 

8.  Fine  sandy  loam  16.  Sandy  clay 

The  meaning  of  these  names  should  be  clear  except  possibly 
those  into  which  the  loam  group  is  divided.  Loam,  as  already 
explained,  refers  to  a  soil  possessing  in  about  equal  amounts 
the  properties  imparted  by  the  various  separates.  If,  how- 
ever, we  have  practically  the  same  condition  but  with  one 


THE  SOIL  PARTICLE 


83 


size  of  particle  predominating,  the  name  of  that  particular 
separate  is  prefixed,  giving  still  more  data  regarding  the  soil 
in  question.  Thus,  a  loam  in  which  clay  is  dominant  will  be 
classified  as  a  clay  loam.  In  the  same  way,  we  may  have  a 
sandy  loam,  silt  loam  and  so  on.  It  is  to  be  noted  that  the 
loams  make  up  half  of  the  class  names.  In  fact,  the  greater 
proportion  of  the  soils  so  far  classified  in  the  United  States 
are  loams,  which  is  fortunate  as  the  loams  in  general  are  more 
favorable  for  crop  production  than  any  of  the  other  class 
groups. 

The  mechanical    analyses    of    some  of  the  more  common 
classes1  are  listed  in  Table  XVI : 


Table  XVI 

Fine 
Gravel 

Coarse 
Sand 

Medium 
Sand 

Fine 
Sand 

Vert 
Fine 
Sand 

Silt 

Clay 

Coarse  Sands .... 

Sands  

Fine  Sands 

Sandy  Loams. . . . 
Fine  Sandy  Loams 
Loams 

12 
2 
1 
4 
1 
2 
1 
2 
1 
0 
1 

31 
15 
4 
13 
3 
5 
2 
8 
4 
2 
3 

19 
23 

10 

12 

4 

5 

1 
8 
4 

1 

2 

20 

37 

57 

25 

32 

15 

5 

30 

14 

4 

8 

6 
11 
17 
13 
24 
17 
11 
12 
13 
7 
8 

7 
7 

7 
21 
24 
40 
65 
13 
38 
61 
36 

5 

5 

4 

12 

12 

16 

Silt  Loams 

Sandy  Clays 

Clay  Loams 

Silty  Clay  Loams 
Clays  

15 

27 
26 
25 
42 

It  is  evident  that  a  mechanical  analysis  of  a  soil  is  nothing 
more  or  less  than  an  expression  of  class,  and  the  inferences 
that  may  be  derived  from  either  are  in  general  the  same.  This 
leads  to  a  consideration  of  class  determination. 

45.  Determination  of  soil  classes. — The  common  method 
of  class  determination  is  that  employed  in  the  field.     It  con- 

1  Whitney,  M.,  The  Use  of  Soils  East  of  the  Great  Plains  Region; 
U.  S.  Dept.  Agr.,  Bur.  Soils,  Bui.  78,  p.  12,  1911. 


84  NATURE  AND  PROPERTIES  OF  SOILS 

sists  in  an  examination  of  the  soil  as  to  color,  an  estimation  of 
its  organic  content,  and,  especially,  a  testing  of  the  ' '  feel ' '  of 
the  soil  in  order  to  decide  as  to  the  class  name.  Probably  as 
much  can  be  judged  as  to  the  texture  and  class  of  a  soil  merely 
by  rubbing  it  between  the  thumb  and  the  fingers  or  in  the 
palm  of  the  hand  as  by  any  other  superficial  means.  This 
method  is  used  in  all  field  operations,  especially  in  soil  survey 
work.  It  really  consists  in  sufficiently  recognizing  the  textural 
composition  of  a  soil  that  the  class  name  may  be  determined.1 

The  accuracy  of  such  a  determination  depends  largely  on 
experience.  Inaccuracies  are  likely  to  occur  in  distinguishing 
between  the  various  finer  grades  of  soil ;  for  this  reason,  more 
nearly  exact  methods  are  necessary  at  times,  especially  in 
checking  soil  survey  work  or  in  carrying  out  investigations  in 
which  absolute  accuracy  is  required. 

As  a  mechanical  analysis  of  a  soil  is  really  a  percentage 
expression  of  texture,  it  presents  an  exact  method  for  class 
determination.  For  detailed  work,  somewhat  complicated 
tables2  have   been    arranged;    but    the    following    diagram 

1  Key  for  the  practical  classification  of  mineral  soils : 

I.    Soils  possessing  the  properties  of  one  size  of 
particle  largely. 

1.  Particles  very  large  Gravel 

2.  Particles  apparent  to  eye;  feel  gritty  and 
non-plastic     Sands 

3.  Particles  very  small ;  soil  very  plastic  when 

wet,  hard  when  dry Clay  or 

Sandy  Clay 

II.    Soils  possessing  the  properties  of  a  number  of 
sizes   of    particles — a   mixture. 

1.  A  fairly  equal  exhibit  of  sandy  and 
clayey  properties  Loam 

2.  A  mixture  but  with  sand  predominating . . .  Sandy  Loam 

3.  A  mixture  but  with  silty  character  dom- 
inant.    The  soil  has  a  floury  or  talc  feel 

and  is  quite  plastic  when  wet Silt  Loam 

4.  A  mixture  but  with  clayey  characters  very 
apparent.  Soil  is  very  plastic  and  ap- 
proaches a  clay  in  character Clay  Loam 

2  Bur.  of  Soils,  Soils  Survey  Field  Boole,  p.  17;  U.  S.  Dept.  Agr.,  Bur. 
Soils,  1906.    Also,  Bur.  Soils,  Bui.  78,  p.  12,  1911. 


THE  SOIL  PARTICLE 


85 


(Fig.  17),  devised  by  Whitney,1  presents  a  simple  method  for 
the  identification  of  a  soil  from  a  mechanical  analysis.  The 
convenience  of  such  a  triangular  representation  is  obvious. 

CLAY 
100 


10  20  50  40   50    60  70   80    9Q  10QSlLT 

PER   CENT 

Fig.  17. — Diagram  for  the  determination  of  class  from  a  mechanical 
analysis.  In  using  the  diagram  the  points  corresponding  to  the 
percentages  of  silt  and  clay  are  located  on  the  silt  line  (abscissa) 
and  clay  line  (ordinate)  respectively.  Perpendiculars  at  these 
points  are  then  projected  inward  until  they  intersect.  The  name 
of  the  compartment  in  which  the  intersection  occurs  gives  the  class 
name  of  the  soil  in  question. 


46.     Soil  survey  classification — soil  type. — The  function 
of  the  soil  survey  is  to  investigate  the  nature  and  occurrence 

1  Whitney,  M.,  The  Use  of  Soils  East  of  the  Great  Plains  Megion; 
U.  S.  Dept.  Agr.,  Bur.  Soils,  Bui.  78,  p.  13,  1911. 


86  NATURE  AND  PROPERTIES  OF  SOILS 

of  soils  in  the  field.  The  soils  thus  studied  are  classified  into 
areas  having  approximately  the  same  crop  relations  and  tillage 
properties.  The  location  of  the  areas  of  each  kind  of  soil  is 
represented  on  an  adequate  base  map,  and  their  character  and 
chief  economic  and  agricultural  relations  are  described  in  a 
printed  report  accompanying  the  soil  map.1     (See  Fig.  18). 

In  classifying  soils  six  primary  factors  «are  considered. 
These,  beginning  with  the  broadest,  are  as  follows:  (1)  tem- 
perature, (2)  precipitation,  (3)  agency  of  formation,  (4)  kind 
of  material,  (5)  special  properties  other  than  texture,  and 
(6)  texture.  It  is  obvious  that  certain  soils  may  be  of  different 
texture  but  alike  in  all  other  ways.  Their  climatic  environ- 
ments, mode  of  formation,  rock  materials,  and  specific  prop- 
erties, such  as  color,  drainage,  organic  condition,  and  lime 
content  may  be  approximately  the  same.  Such  soils  are 
grouped  together  as  series  and  the  series  are  named,  generally 
from  some  town,  county,  or  river  of  the  near  vicinity.  Thus 
we  have  the  Norfolk  series  of  the  Atlantic  coastal  plain;  the 
Cecil  soils  of  the  Piedmont  Plateau ;  the  Ontario  series  arising 
from  the  calcareous  till  of  central  New  York  state  and  the 
Marshall  soils  of  the  loessial  region  of  the  Middle  West.  The 
soils  within  each  series  are  approximately  the  same  except  for 
class  distinction. 

The  soil  type  is  the  unit  of  classification  and  may  be  defined 
as  an  area  of  soil  alike  in  all  characteristics,  including  crop 
productiveness.  Obviously  any  soil  class  of  any  particular 
series  would  be  a  soil  type.  Norfolk  sandy  loam,  Ontario  loam, 
and  Cecil  clay  are  examples  of  how  soil  types  are  designated. 
The  type  designation  is  especially  valuable  in  soil  description 
since  the  series  name  expresses  in  one  word  a  great  number  of 
conditions,  which  otherwise  would  require  detailed  explana- 
tion. The  class  name  establishes  in  addition  the  textural  con- 
dition. 

*For  further  information  consult  one  of  the  numerous  soil  survey 
reports  as  published  by  the  U.  S.  Dept.  Agr.,  Bur.  of  Soils. 


Bui.  60,  Bureau  of  Soils,  U.  S.  Dept.  of  Agricultur 


Dunkirk  Huntington  Dunkirk 


Muck 


loam  gravelly  loam 


loam 


clay 


Fig.  18. — Part  of  the  Madison  County,  New  York,  soil  map  showing  the 
topography  and  drainage  and  the  relation  of  the  various  soil  types 
to  one  another.  The  Volusia  series  arises  from  the  ground  moraine, 
the  Dunkirk  from  glacial  lake  sediments  while  the  Huntington  is 
alluvial.     Note  the  varying  elevation  of  the  muck. 


THE  SOIL  PARTICLE  87 

While  the  principles  of  series  identification  are  too  com- 
plicated to  be  expanded  farther  at  this  time,  enough  has  been 
said  to  establish  the  importance  of  accurate  soil  classification. 
Unless  soils  are  accurately  named  in  soil  survey  work,  the  map 
and  its  accompanying  report  are  useless. 

Soil  texture  and  class  are  thus  the  basis  for  practical  soil 
study,  whether  regarding  some  particular  property  or  a  gen- 
eral condition,  such  as  crop  adaptation.  No  matter  what  the 
phase  of  soil  study  may  be,  texture  and  class  are  sure  to  have 
some  important  influence  and  must  be  considered  in  the  in- 
vestigation. 

47.  Soil  structure. — While  texture  is  of  great  importance 
in  determining  the  general  characteristics  of  a  soil,  it  is  evi- 
dent that  the  arrangement  as  well  as  the  size  of  the  particles 
must  exert  some  influence.  The  term  structure  is  used  to  refer 
to  this  arrangement  or  grouping.  It  is  at  once  apparent  that 
soil  conditions — such,  for  example,  as  air  and  water  move- 
ment, heat  transference,  and  the  like — will  be  as  much  affected 
by  structure  as  by  texture.  As  a  matter  of  fact,  the  great 
changes  wrought  by  the  farmer  in  making  his  soil  better 
suited  as  a  foothold  for  plants  are  structural  rather  than 
changes  in  texture.  The  compacting  of  a  light  soil  or  the 
loosening  of  a  heavy  one  is  merely  a  change  in  the  arrange- 
ment of  the  soil  grains  and  in  the  condition  and  nature  of  the 
colloidal  complexes1  thereof. 

From  the  standpoint  of  size  and  arrangement  of  particles 
there  are  really  two  classes  of  soils,  those  of  single  grain  struc- 
ture and  those  which  are  complex,  the  particles  both  large 
and  small  being  bound  together  by  indefinite  colloidal  com- 
plexes. The  former  condition  is  of  course  best  exemplified 
by  a  sand.  Such  a  soil  is  loose  and  open  with  large  individual 
pore  spaces  and  ready  circulation  of  air  and  water.    The  com- 

1  Material  in  a  colloidal  state  has  a  great  deal  to  do  with  all  soil 
phenomena.  Its  characteristics  and  influence  must  be  kept  constantly  in 
mind  in  soil  study. 


88  NATURE  AND  PROPERTIES  OF  SOILS 

plex  structure  is  best  developed  in  clay.  Here  the  soil  gran- 
ules are  made  up  of  many  particles,  the  colloidal  material  act- 
ing as  a  binding  agent.  Such  a  soil  may  be  loose,  open  and 
friable,  if  granules  of  the  proper  size  and  nature  are  developed. 
On  the  other  hand,  improper  handling  may  run  the  complexes 
together  and  an  impervious  and  puddled  condition  may  result. 
The  sand  will  obviously  permit  of  no  very  great  structural 
change,  while  the  clay  can  be  modified  very  materially  by 
certain  field  manipulations. 

The  ideal  structural  condition  is  most  likely  to  occur  in  a 
loam  soil.  In  such  a  soil  some  of  the  particles  are  large  and 
function  separately;  others  are  medium  in  size  and  tend  to 
form  the  nuclei  around  which  smaller  particles,  both  colloidal 
and  non-colloidal,  may  cluster  to  form  granules,  or  aggregates. 
There  are  thus  a  few  large  pore  spaces  which  facilitate  drain- 
age, and  numberless  small  openings  in  which  water  is  retained. 
Air,  therefore,  finds  easy  movement  and  sanitation  is  pro- 
moted. In  promoting  such  a  condition  the  *TffgTlifl  ffl  fitter 
plays  an  important  part.  It  usually  exists  as  a  dark,  partially 
decayed  material,  often  colloidal  in  nature.  It  pushes  apart 
the  grains  and  lightens  the  soil,  and  contributes  much  in  bring- 
ing about  the  loamy  condition  so  favorable  to  plant  develop- 
ment. It  is  a  valuable  addition  also  on  account  of  its  water- 
holding  capacity  and  its  nitrogen  content. 

48.  Specific  gravity  of  soils. — The  texture,  as  well  as  the 
structure  of  a  soil,  has  considerable  influence  on  certain  phys- 
ical conditions  other  than  those  already  mentioned.  One  of 
these  is  weight.  The  weight  of  a  soil  is  determined  by  two 
factors :  the  weight  of  the  individual  particles  and  the  amount 
of  the  space  occupied  by  the  soil  material.  The  former  is 
determined  by  the  chemical  and  mineralogical  character  of 
the  particles,  the  latter  by  their  structural  arrangement.  Thus, 
if  the  soil  particles  are  heavy  and  the  soil  is  compact,  the 
weight  of  any  given  volume,  a  cubic  foot  for  example,  will  be 
high. 


THE  SOIL  PARTICLE 


89 


The  specific  gravity1  of  a  soil  is  obviously  the  average  spe- 
cific gravity  of  the  particles.  It  is  unaffected  by  the  structure, 
remaining  the  same  whether  the  soil  is  loose  and  open  or  com- 
pact and  unaerated.  Although  a  great  range  is  observed  in 
the  specific  gravities  of  the  common  soil  minerals2,  the  spe- 
cific gravity  of  a  purely  mineral  soil  varies  between  the  nar- 
row limits  of  2.6  and  2.7.  This  occurs  because  quartz  and 
feldspar,  whose  specific  gravities  are  about  2.65  and  2.57, 
respectively,  usually  make  up  the  bulk  of  the  mineral  portion 
of  most  soils.  The  fineness  of  the  particles  seems  to  have  no 
appreciable  effect  on  specific  gravity  as  shown  by  the  follow- 
ing data  from  Whitney  and  Smith3: 


Table  XVII 

SPECIFIC  GRAVITY  OF  SOIL  SEPARATES 

Separates 

Whitney 

Smith 

Fine  gravel 

2.64 
2.65 
2.64 
2.65 

2.68 
2.69 

2.83 

2.67 

Coarse  sand 

2.64 

Medium  sand 

2.64 

Fine  sand 

2.69 

Very  fine  sand 

2.66 

Silt 

2.65 

Clay 

2.66 

1  Specific  gravity  is  expressed  as  a  ratio  of  the  weight  of  any  volume 
of  a  substance  to  the  weight  of  an  equal  volume  of  some  other  substance 
taken  as  a  standard  unit.  Liquids  and  solids  are  usually  compared  with 
water  at  its  maximum  density  (4°  C). 

2  The  specific  gravities  of  some  of  the  common  soil  minerals  are  as 
follows: 


Quartz 2.60-2.70 

Orthoclase     2.57 

Plogioclase   2.62-2.76 

Muscovite 2.76-3.00 

Biotite    2.70-3.10 

Hornblende    3.05-3.47 

Augite     3.20-3.60 


Apatite    3.20 

Kaolinite    2.60-2.63 

Serpentine    2.50-2.65 

Chlorite 2.65-2.92 

Epidote 3.25-3.50 

Hematite    4.90-5.30 

Limonite   3.60-4.00 


3  Whitney,  M.,  Some  Physical  Properties  of  Soils;  U.  S.  Dept.  Agr., 
Weather  Bur.,  Bui.  4,  1892.  Smith,  Alfred,  Eelation  of  the  Mechanical 
Analysis  to  the  Moisture  Equivalent  of  Soils;  Soil  Sci.,  Vol.  IV,  No.  6. 
p.  472,  Dec,  1917. 


90  NATURE  AND  PROPERTIES  OF  SOILS 

The  only  marked  variation  here  observed  is  in  the  clay 
separates  of  the  first  column.  This  may  be  due  to  the  concen- 
tration of  the  iron-bearing  silicates  in  this  grade  and  would 
thus  be  an  apparent  rather  than  a  real  variation. 

Only  one  condition  may  vary  the  specific  gravity  of  any 
soil.  This  is  the  quantity  of  organic  matter  present.  As  the 
specific  gravity  of  organic  matter  usually  ranges  from  1.2  to 
1.7,  the  more  that  is  present  the  lower  will  be  the  figure  for 
any  given  soil.  A  purely  organic  soil,  such 
as  muck,  presents  a  variable  specific  grav- 
ity ranging  from  1.5  to  2.0,  according  to 
the  amount  of  inorganic  wash  it  has  re- 
ceived from  external  sources.  Some  highly 
organic  mineral  soils  may  drop  as  low  as 
2.3.  Nevertheless,  for  general  calculations, 
the  average  arable  soil  may  be  considered 
to  have  a  specific  gravity  of  about  2.65. 
The  specific  gravity  of  a  soil  is  generally 

Fig.    19—  Drawing     determined  by  means  of  a  picnometer,  a 
snowing  the  type     ,        ,     ~ 

0  f      picnometer    bottle  fitted  with  a  perforated  ground-glass 

generally  used  in     stopper    and    accurately    calibrated    (Fig. 
determining     the     .,,..       -~  .  ,  .  ,        »    , 

specific  gravity  of     I9)  •    BY  comparing  the  weight  of  the  total 

soil.  The  ground-     water  held  by  the  bottle,  usually  50  cubic 

glass    stopper    is  A.       .  ..*    ,,  ... 

perforated.  centimeters,  with  the  weight  of  the  water 

when  any  given  amount  of  dry  soil,  say  5 
grams,  is  present  in  the  bottle,  the  weight  of  the  water  dis- 
placed by  the  soil  can  be  determined  and  the  specific  gravity 
calculated  therefrom.1 

1  Below  will  be  found  a  sample  calculation : 

Weight  of  pienometer   23.257  grs. 

Volume  of  picnometer 50         cc. 

Wt.  of  picnometer  -f-  5  grs.  soil  +  X  grs.  water.  .  .  .  76.347  grs. 

Wt.  of  picnometer  +  5  grs.  soil 28.257  grs. 

Wt.  of  X  grs.  water 48.090  grs. 

Water  displaced  (50  —  48.09)    1.910  grs. 

Specific  gravity  =    '      =  2.61  -f 


THE  SOIL  PARTICLE  91 

49.  Volume  weight  of  soils. — The  actual  weight  of  dry 
soil  in  any  given  volume  is  generally  expressed  by  volume 
weight,,  a  figure  indicating  the  number  of  times  heavier  the 
dry  soil  is  than  the  water  that  wiUoccupy  the  same  soil  vol- 
ume. Thus,  if  the  dry  soil  in  a  cubic  foot  of  space  weighs 
99.8  pounds,  the  volume  weight  would  be  99.8-^62.42  or  1.6. 
The  volume  weight  differs  from  specific  gravity  in  that  it 
compares  the  weight  of  the  dry  soil  to  the  weight  of  water 
that  will  occupy  the  total  soil  volume — that  is,  the  space 
usually  filled  by  soil  particles,  soil  air,  and  soil  water.  Specific 
gravity,  however,  compares  the  weight  of  the  dry  soil  to  that 
of  water  that  will  occupy  only  the  volume  of  the  particles 
alone,  taking  no  consideration  of  the  normal  pore  space.  It 
is  consequently  always  the  higher  figure.1 

This  volume  weight  figure  depends  on  the  texture  of  the 
soil,  the  structure  and  the  amount  and  condition  of  the  organic 
matter.  The  particles  of  sandy  soils  always  tend  to  lie  in 
close  contact,  thus  increasing  the  weight  of  soil  to  a  given 
volume.  The  particles  of  the  finer  soils,  such  as  silt  loams, 
clay  loams,  and  clays,  on  the  other  hand,  being  smaller  and 
lighter,  do  not  lie  so  closely  together.  A  greater  total  pore 
space  is,  therefore,  usually  present  in  the  finer  soils  and  the 
volume  weight  is  correspondingly  lowered.  Mineral  soils  may 
range  in  volume  weight  from  1.10  to  1.35  for  clay  to  1.55  to 
1 .  70  for  sand.2  The  influence  of  texture  on  the  volume  weight 
is  thus  evident. 

The  structural  and  organic  condition  of  soils  often  pro- 
duces wide  variation  in  volume  weight.    When  a  soil  is  loos- 

1  As  a  soil  is  compacted,  its  volume  weight  increases  due  to  the  increase 
volume  occupied  by  the  soil  particles  and  the  corresponding  decrease  in 
pore  space.  If  it  were  possible  to  compact  a  soil  to  a  completely  solid 
condition,  its  volume  weight  would  approach  its  specific  gravity  as  a 
limit.  Specific  gravity  represents,  therefore,  100  per  cent,  soil  particles. 
Volume  weight  in  comparison  indicates  the  proportion  of  space  occupied 
by  the  soil  particles. 

2  Sandy  soils  are  commonly  spoken  of  as  light  soils,  while  clays  are 
called  heavy.  Such  usage  refers  to  working  properties  and  has  no 
reference  to  actual  weights. 


92  NATURE  AND  PROPERTIES  OF  SOILS 

ened  through  tillage,  it  becomes  lighter  for  any  given  volume. 
The  addition  of  organic  matter  has  the  same  effect,  since  the 
particles  are  spread  wider  apart  and  the  air  and  water  spaces 
increased.  The  specific  gravity  figure  of  a  sandy  loam  of  1.55 
may  readily  be  lowered  to  1.45  by  an  increase  of  organic 
material.  Some  loams  high  in  organic  matter  may  drop  as 
low  as  1.1  in  specific  gravity  while  muck  often  reaches  the 
low  figure  of  .40. 

In  the  field  the  volume  weight  of  a  soil  may  be  estimated  by 
driving  a  cylinder  of  known  volume  into  the  ground  and  ob- 
taining thereby  a  core  of  natural  soil.  By  weighing  the  soil 
and  then  determining  the  amount  of  water  that  it  holds,  the 
amount  of  absolutely  dry  soil  may  be  ascertained.  Dividing 
this  by  the  weight  of  an  equal  volume  of  water  gives  the  figure 
for  volume  weight.1 

A  laboratory  determination  may  be  made  by  putting  the 
soil  into  a  receptacle  of  known  volume  and  weighing  it.  From 
the  weight  of  the  absolutely  dry  soil  and  the  weight  of  an 

1  The  rubber  tube  method  has  proven  very  convenient  for  the  field  de- 
termination of  volume  weight.  A  hole  is  bored  in  the  soil  to  the  required 
depth  by  a  specially  constructed  auger,  the  soil  being  carefully  removed 
and  later  oven  dried.  A  very  thin-walled  tubular  rubber  bag  of  the  size 
of  the  auger  hole  is  carefully  inserted  in  the  hole  previously  bored.  The 
tubular  bag  is  then  filled  with  water  flush  with  the  surface  of  the  soil. 
The  water  is  measured  and  the  volume  of  the  soil  removed  is  thus  de- 
termined. Knowing  the  weight  of  dry  soil  and  its  original  volume,  the 
volume  weight  may  be  calculated.  The  experimental  error  of  the  method 
is  rather  low. 

Israelsen,  O.  W.,  A  New  Method  of  Determining  Volume  Weight; 
Jour.  Agr.  Ees.,  Vol.  XIII,  No.  1,  pp.  28-35,  April,  1918. 

The  paraffin-immersion  is  valuable  with  heavy  soils.  Small  pieces  of 
soil  are  dried,  weighed  and  then  coated  very  thinly  with  paraffin,  just 
sufficiently  to  prevent  the  entrance  of  water,  yet  not  enough  to  intro- 
duce serious  experimental  error.  The  weight  of  the  water  displaced  by 
a  number  of  such  pieces  may  be  determined  easily  by  the  use  of  a 
graduated  cylinder. 

Shaw,  C.  F.,  A  Method  for  Determining  the  Volume  Weight  of  Soil 
in  Field  Condition;  Jour.  Amer.  Soc.  Agron.,  Vol.  IX,  No.  1,  pp.  38-42, 
1917.  See  also,  Trnka,  R.,  Eine  Studie  uber  einige  physitcalishchen 
Eigenscliafteii  des  Bodens;  Internat.  Mitt,  of  Bodenkunde,  Bd.  IV,  Heft 
4-5,  S.  363-380,  1914. 


THE  SOIL  PARTICLE  93 

equal  volume  of  water,  the  volume  weight  may  be  calculated. 
This  method  will  give  only  approximate  results,  however,  as 
the  structural  relationships  are  more  or  less  artificial.1 

50.  Actual  weight  of  soil. — When  the  volume  weight  of 
a  soil  is  known,  its  weight  in  pounds  to  the  cubic  foot  may  be 
found  by  multiplying  by  62.42.  Soils  may  vary  in  weight 
from  68  to  80  pounds  for  clays  and  silts  to  100  to  110  pounds 
for  sands.  The  greater  the  organic  content,  the  less  is  this 
weight  to  the  cubic  foot.  A  muck  soil  often  weighs  as  little 
as  25  or  30  pounds.  This  weight,  of  course,  is  for  absolutely 
dry  soil  and  does  not  include  the  water  present,  which  may  be 
much  or  little,  according  to  circumstances. 

The  actual  weight  of  soil  may  also  be  expressed  in  acre-feet. 
An  acre-foot  of  soil  refers  to  a  volume  of  soil  one  acre  in 
extent  and  one  foot  deep.  In  the  same  way  we  may  have 
an  acre-eight-inches  or  an  acre-six-inches.  The  weight  of  an 
acre-foot  of  soil  usually  varies  from  3,500,000  to  4,000,000 
pounds.  The  standard  usually  adopted  is  2,000,000  pounds, 
being  the  weight  of  average  soil  to  a  depth  of  62/3  inches. 
The  value  of  knowing  the  actual  weight  of  a  soil  lies  in  the 
possibility  of  calculating  thereby  the  amount  of  water,  the 
amount  of  organic  matter,  or  the  actual  number  of  pounds  of 
the  mineral  constituents  present  in  the  soil.  Such  informa- 
tion affords  another  means  of  comparing  two  soils. 

51.  Pore  space  of  soil. — The  pore  space  of  soil  is  occu- 
pied by  air  and  water  in  constantly  varying  proportions.  The 
amount  of  this  pore  space  is  determined  by  the  texture  and 
the  structure  of  the  soil.    As  already  emphasized,  the  coarser 

1  A  comparison  of  the  four  methods  is  given  by  Israelsen,  O.  W.,  A 
New  Method  for  Determining  Volume  Weight;  Jour.  Agr.  Kes.,  VoL 
XIII,  No.  1,  p.  32,  1918. 

Average  Volume  Weight  of  Tehama  Clay  to  a  Depth  of  60  Inches. 

Laboratory  method  on  disturbed  soil 1.35  ±  .008 

Eubber  tube  method  1.74  ±  .010 

Iron  cylinder  method   1.73 

Paraffin-immersion  method 1.73  ±  .035 


94  NATURE  AND  PROPERTIES  OF  SOILS 

soils  are  heavy  due  to  the  close  contact  of  the  particles,  while 
the  finer  soils  are  much  lighter  due  to  the  tendency  of  the 
small  particles  to  resist  compaction.1  This  means  that  soils 
such  as  sands  and  sandy  loams  contain  less  pore  space  than 
silt  loams,  clay  loams,  and  clays.  While  the  heavier  soils  have 
more  combined  air  and  water  space,  the  individual  spaces  are 
much  smaller  than  in  the  sands,  which  accounts  for  the  slow 
air  and  water  drainage  in  the  former  and  the  ease  with  which 
such  phenomena  take  place  in  the  lighter  soils. 

A  very  simple  formula  may  be  used  to  calculate  pore  space, 
providing  the  specific  gravity  and  volume  weight  are  known. 
It  is  subject  to  considerable  inaccuracy,  however,  because 
of  the  presence  of  colloidal  matter,  the  exact  influence  of  which 
cannot  be  determined. 

rrf     t»  a  -.™  /V0l.    Wt.         100  V 

%  Pore  Space  =  100  —  1 x  -—  1 

A  soil  having  a  volume  weight  of  1.6  and  a  specific  gravity 
of  2.6  has,  according  to  this  formula,  38.5  per  cent,  of  pore 
space.  A  soil  in  which  the  above  figures  are  1.1  and  2.5, 
respectively,  possesses  56  per  cent,  of  air  and  water  space. 

The  following  figures  taken  from  King  3  illustrate  the  rela- 
tion that  texture  and,  to  a  certain  extent,  structure  also  occu- 
pies in  relation  to  soil  pore  space : 

1  Sandy  soils  are  generally  spoken  of  as  loose,  while  clays  are  called 
compact.  The  term  compact  is  thus  used  in  the  sense  of  hard,  unyielding, 
stiff,  or  impenetrable,  and  does  not  indicate  that  the  pore  space  of  clay 
is  less  than  that  of  a  sandy  soil. 

3  It  has  already  been  explained  in  a  previous  footnote  (see  under 
volume  weight)  that  the  specific  gravity  of  a  soil  represents  100  per 
cent,  soil  material  or  the  weight  of  absolutely  solid  soil.  Volume  weight 
indicates  in  comparison  thereto,  the  soil  material  actually  present.  The 
ration  of  the  specific  gravity  to  the  volume  weight  when  multiplied  by 
100  becomes  the  percentage  of  the  soil  volume  occupied  by  the  soil 
particles. 

3  King,  F.  H.,  Physics  of  Agriculture;  published  by  the  author,  Madi- 
son, Wisconsin,  1910. 


THE  SOIL  PARTICLE  95 

Table  XVIII 

PERCENTAGE   PORE   SPACE  IN   SOILS   OF   DIFFERENT   TEXTURE 

Sandy  soil 32.5 

Loam   34 . 5 

Heavy  loam  44 . 1 

Loamy  clay    45 . 3 

Clayey  loam 47 . 1 

Clay    48.0 

Heavy  clay 52 .9 

The  pore  space  in  a  normal  soil  is  occupied  by  water  and 
air.  If  the  water  content  is  low,  the  air  space  is  large,  and 
vice  versa.  Thus  the  relationships  of  the  total  pore  space 
and  the  size  of  the  individual  spaces  to  the  amount  of  air  and 
water  contained,  to  their  movement  through  the  soil,  to  soil 
sanitation,  to  root  extension,  to  bacterial  action,  and  to  crop- 
ping conditions  in  general,  become  apparent.  It  is  the  regu- 
lation of  this  pore  space  that  is  really  important  in  any  struc- 
tural consideration.  The  effect  on  plant  growth  of  a  change 
of  pore  space  is  the  only  test  of  its  advisability. 

52.  Soil  particles — their  number  and  surface  exposed. 
— Since  soil  particles  run  to  extremely  small  diameters,  the 
number  in  any  given  volume  is  very  large,  especially  when 
fine-textured  soils  are  considered.  However,  any  calculation 
of  the  number  of  particles  present  in  a  soil  is  open  to  great 
inaccuracy  j  first,  because  it  is  impossible  to  get  a  correct  fig- 
ure for  the  average  diameter  of  the  particles  of  any  soil  or  of 
the  various  groups  of  separates  that  go  to  make  it  up;  and, 
secondly,  because  it  must  be  assumed  in  the  calculation  that 
the  particles  are  spherical.  The  presence  of  colloidal  matter, 
especially  in  the  heavier  soil  types,  introduces  an  error  the 
magnitude  of  which  must  be  very  great.  Nevertheless,  such 
a  calculation,  even  if  very  inaccurate,  gives  some  idea  as  to 
the  immense  number  of  grains  that  are  present  even  in  the 


96  NATURE  AND  PROPERTIES  OF  SOILS 

coarser  soils.    A  few  figures  are  given  in  Table  XIX  for  some 
of  the  average  soil  classes *  established  by  the  Bureau  of  Soils : 

Table  XIX 

APPROXIMATE  NUMBER  OF  PARTICLES  TO  A  GRAM  OF  VARIOUS  SOIL 

CLASSES  2 


Soil  Class 

Number  of  Particles 
to  the  Gram 

Sands 

2,287,000,000 

5,483,000,000 

7,332,000,000 

11,877,000,000 

19,177,000,000 

Sandy  loams 

Loams 

Clay  loams 

Clays 

An  important  property  of  the  surface  of  the  grains  is  the 
tendency  toward  the  retention  of  soluble  material  in  a  par- 
tially or  wholly  available  condition  for  plant  use.  This  power, 
designated  as  absorption,  is  exhibited  to  a  high  degree  by 
fine  soils,  in  which  the  individual  pore  spaces  are  small  and 
the  amount  of  surface  exposed  is  large,  due  to  the  presence 
of  considerable  colloidal  matter.  This  capacity  is  an  especially 
important  factor  in  the  economical  use  of  fertilizer  salts.  Ab- 
sorption may  also,  by  bringing  materials  into  closer  contact, 
hasten  or  retard  certain  chemical  actions.    Changes  may  thus 

*The  mechanical  analyses  of  these  particular  classes  are  given  on 
page  83. 

"The  number  of  particles  in  any  soil  sample  may  be  arrived  at  from 
a  mechanical  analysis  and  the  diameters  that  limit  each  group.  Using 
the  average  diameter  of  each  group  together  with  the  percentage  of 
the  groups  in  a  given  sample,  the  number  of  particles  may  be  calculated 
by  the  following  formula: 

,T      .         «        . .  ,                       ,      M      .,      Weight  of  sample  in  grams 
Number  of  particles  in  a  sample  of  soil  = —     r  fa ■ 

-I/O  j^  U     X   u.Ot) 

The  formula  1/6  jr,  D3  is  that  used  for  determining  the  volume  of  a 
sphere,  the  diameter  in  this  case  being  expressed  in  centimeters.  When 
multiplied  by  the  average  specific  gravity  of  soil  particles  the  weight  of 
an  average  particle  is  obtained  in  grams.  In  the  above  calculations, 
2.7  was  used  instead  of  2.65. 


THE  SOIL  PARTICLE 


97 


be  expected  to  go  on  in  the  soil  that  would  not  take  place  in 
the  laboratory  beaker.  The  relation  of  this  absorption  to  bac- 
terial activity  also  cannot  be  overlooked. 

The  minerals  of  the  soil  are  all  very  resistant  to  solution; 
if  they  were  not,  they  would  long  ago  have  been  leached  away. 
Such  materials,  while  almost  insoluble  under  ordinary  cir- 
cumstances, allow  appreciable  amounts  of  nutrients  to  appear 
in  the  soil  solution,  because  of  the  immense  amount  of  surface 
exposed,  although  the  specific  solubility  remains  the  same. 

In  order  to  present  some  idea  of  the  internal  surface  of 
ordinary  soils,  a  few  figures  are  given  on  the  same  soil  classes 
for  which  the  number  of  particles  have  already  been  calcu- 
lated : 

Table  XX 

APPROXIMATE  INTERNAL  AREA  OF  SEVERAL  AVERAGE  SOIL 

CLASSES  1 


Soil  Class 

Square 

Inches 

per  Gram 

Square 

Feet  per 

Pound 

Acres  per 
Acre-Foot  of 
3,500,000  lbs.- 

Sands 

89 
213 
294 
430 
653 

280 

671 

926 

1354 

2057 

22,549 

53,965 

74,410 

108,830 

165,270 

Sandy  loams 

Loams 

Clays  loams 

Clays 

While  these  figures  are  as  grossly  inaccurate  as  those  re- 
garding the  number  of  particles,  they  tend  to  emphasize  the 
tremendous  internal  surface  possessed  by  even  the  coarser 
soils.  The  data  presented  for  an  acre-foot  of  soil,  while  al- 
most too  large  for  adequate  comprehension,  are  probably 
much  too  low.  It  is  not  to  be  wondered  at  that  the  slowly 
soluble  minerals  are  able  to  supply  sufficient  nutrients  to  the 

1When  the  approximate  number  of  particles  and  their  sizes  in  any 
given  weight  of  soil  are  known,  the  internal  surface  may  be  calculated 
by  the  following  formula: 

Surface  =  jt  D2  X  number  of  particles. 


98  NATURE  AND  PROPERTIES  OP  SOILS 

crop  growing  on  the  soil,  when  such  a  large  amount  of  sur- 
face  is  continually   available  for  chemical   action. 

53.  Resume. — The  discussion  of  the  soil  particle  as  to  its 
size,  its  classification,  its  chemical  characteristics,  and  its 
mineralogical  peculiarities  is  undoubtedly  important.  Im- 
portant also  are  the  specific  physical  properties  which  arise 
because  of  textural  and  structural  make-up,  such  as  specific 
gravity,  volume  weight,  pore  space,  and  immense  internal 
surface.  These  phases,  however  interesting  in  themselves, 
must  not  be  studied  so  closely  as  to  prevent  their  broad  and 
vital  plant  correlations  from  becoming  evident.  None  of  the 
transformations  concomitant  with  normal  crop  production 
takes  place  in  the  soil  without  definite  and  widespread  co- 
operation. The  study  of  the  soil  particle  is,  therefore,  more 
than  a  consideration  of  a  few  interesting  physical  and  chem- 
ical phenomena.  From  such  investigations  have  been  devel- 
oped and  perfected  the  broad  principles  which  govern  suc- 
cessful soil  management  and  economical  food  production. 


pf 


"**' 


v*^ 


CHAPTER  V 
THE  ORGANIC  MATTER  OF  THE  SOIL 


One  of  the  essential  differences  between  a  soil  and  a  mass 
of  rock  fragments  lies  in  the  organic  content  of  the  former. 
Organic  matter  is  necessary  in  order  that  mineral  material 
may  become  a  soil  and  that  it  may  grow  crops  successfully. 
The  physical  condition  of  soils  depends  largely  on  the  pres- 
ence of  organic  matter  and  chemical  reaction  is  greatly  ac- 
celerated by  its  decay. 

In  the  process  of  soil  formation  the  addition  of  organic 
materials  is  more  or  less  a  secondary  step.  In  residual  debris 
the  amount  of  organic  matter  held  by  the  growing  soil  in- 
creases as  the  process  of  weathering  goes  on ;  in  glacial  soils, 
however,  the  matrix  or  skeleton  of  the  soil  is  already  formed 
before  there  is  an  opportunity  for  organic  matter  to  become 
incorporated  in  it.  The  final  result  from  the  mixing  of  min- 
erals and  their  weathered  and  altered  products  with  the 
decayed  or  partially  decayed  organic  matter  that  is  sure  to 
accumulate,  is  a  mass  much  more  complicated  than  either 
of  the  original  constituents.  The  complexity  of  the  average 
soil  has  already  been  sufficiently  stressed. 

54.  The  source  of  soil  organic  matter1  and  the  char- 
acter of  plant  tissue. — The  source  of  practically  all  soil 

1  The  soil  organic  matter  includes  not  only  all  compounds  contained 
in  the  original  vegetable  and  animal  tissues  but  also  those  existing  in 
the  partially  decayed  portions  of  such  material.  Carbon  dioxide,  methane 
and  like  compounds  are  usually  not  considered  as  a  part  of  the  soil 
organic  matter.  In  this  respect,  the  above  definition  is  narrower  than 
that  for  organic  chemistry,  which  is  the  chemistry  of  carbon  compounds. 
For  a  very  good  review  of  literature  on  soil  organic  matter,  see  Morrow, 
C.  A.,  The  Organic  Matter  of  the  Soil:  A  Study  of  the  Nitrogen  Distri- 
bution in  Different  Soil  Types;  Dissertation,  Univ.  Minn.,  1918. 

99 


100         NATURE  AND  PROPERTIES  OF  SOILS 

organic  matter  is  plant  "[issue.1  Some  of  this  matter  ac- 
cumulates from  the  above-ground  parts  of  plants  that  have 
died  and  fallen  down  to  become  mixed  with  the  surface  soil; 
the  remainder  is  a  result  of  root  extension  and  subsequent 
decay.  The  organic  matter  of  the  surface  soil  is  derived  from 
the  tops  and  the  roots  of  plants  growing  on  it,  while  that  of 
the  subsoil  is  very  largely  a  result  of  root  extension  and  sub- 
sequent  decomposition. 

Since  soil  organic  matter  has  its  origin  very  largely  from 
the  higher  plants,  it  is  advisable  to  consider  the  general  chem- 
ical nature  of  such  material.2  About  75  per  cent,  of  average 
green  plant  tissue  is  water.  The  dry  matter  is  made  up  of 
carbon,  oxygen,  hydrogen,  and  mineral  material  in  the  ap- 
proximate ratio  of  6,  5,  1  and  1  respectively.  The  preponder- 
ant elements  of  normal  plant  tissue  are  evidently  carbon, 
oxygen,  and  hydrogen.     (See  Fig.  20.) 

It  is  usual  in  classifying  the  compounds  in  plants  to  group 
them  under  the  following  heads:  (1)  carbohydrates,  (2) 
fixed  oils  and  waxes,  (3)  volatile  oils  and  resins,  (4)  organic 
acids  and  their  salts,  and  (5)  nitrogenous  compounds.3     The 

1  It  must  not  be  inferred  that  higher  plants  are  the  only  source  of  soil 
organic  matter.  Assuming  that  the  weight  of  one  bacterial  cell  is 
.000,000,002  of  a  milligram  and  that  in  each  gram  of  a  normal  fertile 
soil,  weighing  2,000,000  pounds  to  an  acre-seven  inches,  there  are 
100,000,000  of  such  organisms,  the  weight  of  bacteria  alone  would  be 
400  pounds  to  the  surface  acre.  This  is  a  very  conservative  estimate, 
800  pounds  probably  being  more  nearly  correct.  Considering  the  molds, 
fungi,  algae,  actinomycetes,  insects,  and  earthworms,  there  are  probably 
2000  pounds  of  living  material  in  every  acre  of  normal  soil  exclusive 
of  plant  roots.  These  organisms  in  their  functioning  supply  no  insig- 
nificant portion  of  the  soil  organic  matter. 

aFor  a  fuller  discussion  see:  Ingle,  Herbert,  Manual  of  Agricultural 
Chemistry,  Chap.  X,  London,  1913.  Also,  Stoddard,  C.  W.,  The  Chem- 
istry of  Agriculture,  Chap.  Ill,  Philadelphia  and  New  York,  1915. 
Also,  Thatcher,  R.  W.,  The  Chemistry  of  Plant  Life,  New  York,  1921. 

3 1.  Carbohydrates — Sugars,  starch,  cellulose,  legnin,  inulin,  gums, 
pectins,  and  pentosans. 

II.  Fixed  oils  and  waxes — Castor  oil,  corn  oil,  cottonseed  oil,  linseed 
oil,  and  the  like. 

III.  Volatile  oils  and  resins— Oil  of  mustard,  of  cloves,  of  pepper- 
mint, etc.    Rosin,  myrrh,  balsam,  etc. 


THE  ORGANIC  MATTER  OF  THE  SOIL         101 

mineral  matter  or  so-called  ash  exists  as  a  part  of  the  com- 
pounds listed  under  these  headings.  The  carbohydrates,  hav- 
ing the  general  formula  of  Cx(H20)n  include  such  compounds 
as  starch,  cellulose,  dextrose,  glucose,  cane  sugar,  and  the  like. 
The  fats  and  oils  may  be  represented  in  plants  by  such  glycer- 
ides  as  butyrin,  stearin,  olein,  palmitin,  while  many  acids  of 
an  organic  nature  exist  especially  in  fruits  and  vegetables. 


Fig.  20. — Diagram  showing  the  general  composition  of  green  plant  tissue. 
The  nitrogen  which  is  generally  less  than  .5  per  cent,  is  included 
with  the  ash  in  the  above  diagram.     (After  Stoddard.) 

Of  the  five  groups,  however,  the  nitrogenous  compounds  are 
probably  the  most  complicated  as  they  carry  not  only  carbon, 
hydrogen,  oxygen,  and  nitrogen,  but  also  mineral  elements 
such  as  sulfur,  phosphorus,  calcium  and  iron.  They  are  com- 
pounds of  high  molecular  weight  and  many  are  of  unknown 

IV.  Organic  acids  and  their  salts — Citric  acid,  malic  acid,  tannic  acid, 
tartaric  acid,  and  the  like. 

V.  Nitrogenous  compounds — Nitrates,  ammonia,   amides,   amino-acids, 
alkaloids,  and  proteins. 


102         NATURE  AND  PROPERTIES  OF  SOILS 

constitution.  Simple  proteins,  such  as  albumin,  globulin,  pro- 
tamins,  and  others,  are  found  in  plants,  besides  certain  de- 
rived proteins  such  as  proteosis  and  peptones.  In  addition 
to  all  these,  there  is  a  host  of  other  nitrogenous  compounds 
that  have  no  small  influence  on  the  composition  of  the  soil 
organic  matter.1 

It  is  also  necessary  to  consider  that  certain  portions  of  the 
cell  contents  and  cell  walls  are  in  a  collodial  state.  Such  a 
condition  is  important  as  the  translocation  of  dissolved  sub- 
stances from  soil  to  plant  and  from  cell  to  cell  depend  largely 
on  their  diffusibility  through  colloidal  membranes. 

It  is  evident  even  from  this  brief  discussion  that  the  chem- 
ical character  of  plant  tissue  is  far  from  simple.  The  degra- 
dation of  such  material,  especially  in  the  presence  of  com- 
plex mineral  products,  generally  gives  rise  at  first  to  com- 
pounds no  simpler;  in  fact,  the  chances  are  that  the  result- 
ing compounds  will  be  much  more  complicated.  It  is  only 
later  in  the  processes  of  decomposition  that  simple  products 
result. 


1  Crops  are  usually  analyzed  for  six  constituents — water,  ash,  crude 
protein,  crude  fiber,  nitrogen  free  extract,  and  crude  fat.  Water  is 
determined  by  drying  the  sample  at  the  temperature  of  boiling  water. 
By  burning  a  sample  of  the  plant  tissue  until  all  of  the  organic  matter 
has  been  driven  off,  the  percentage  of  mineral  matter  may  be  found. 
Crude  protein  is  obtained  by  multiplying  the  figure  for  total  nitrogen 
by  6.25.  Crude  fat  is  found  by  extracting  the  dry  plant  tissue  with 
ether,  while  the  crude  fiber  is  that  which  remains  of  the  fat-free  material 
after  treatment  with  both  dilute  sulfuric  acid  and  dilute  sodium  hydrox- 
ide solutions.  Nitrogen-free  extract  is  the  difference  between  the  sum 
of  the  above  constituents  and  100  per  cent.  Below  are  four  typical 
analyses: 


Crop 

Water 
% 

Ash 

% 

Crude 
Protein 

% 

Crude 

Fiber 
% 

Nitrogen 

Free 

Extract 

% 

Crude 

Fat 

% 

Alfalfa   (green)    . . . 
Lettuce   (fresh)    . . . 

Wheat  (grain)    

Timothy  (hay) 

71.8 
94.7 
10.5 
13.2 

2.7 

.9 

1.8 

4.4 

4.8 

1.2 

11.9 

5.9 

7.4 

.7 

1.8 

29.0 

12.3 

2.2    . 
71.9 
45.0 

1.0 

.3 

2.1 

2.5 

THE  ORGANIC  MATTER  OF  THE  SOIL         103 

55.  Decomposition1  of  organic  matter  in  soils. — While 
the  general  trend  of  organic  degradation  in  soils  is  towards 
simplification,  the  process  is  by  no  means  a  progressive  one. 
Many  products  are  built  up  that  are  much  more  complex 
than  the  original  tissue.  Most  of  the  fermentation  and  putre- 
faction is  due  to  that  great  group  of  organisms  called  bacteria, 
although  molds,  fungi,  and  the  like  also  are  important.  The 
action  of  these  organisms  may  be  direct,  but  is  more  likely 
to  be  enzymic.2  A  cycle  is  therefore  set  up,  in  which  the 
higher  plants  and  animals  are  occupied  in  building  up,  while 
bacteria  are  tearing  down  and  reducing  the  residue  of  plant 
action  to  simple  forms,  such  as  can  be  ultimately  utilized  again 
in  plant  nutrition.  The  importance  of  soil  organisms  is  thus 
evident,  and  the  encouragement  of  their  growth  and  function 
is  clearly  a  part  of  good  soil  management.     (See  Fig.  21.) 

When  the  complex  molecules  that  make  up  plant  tissue 
break  down,  they  split  along  definite  lines  of  cleavage,  de- 
pending on  the  structure  of  the  original  molecule.  These 
bodies,  which  are  usually  simpler  in  nature  than  those  from 
which  they  have  sprung,  are  called  cleavage  products,  and 
without, a  doubt  their  appearance  is  the  first  step  in  organic 
decomposition.  These  compounds  are  subject  to  still  further 
change,  and  because  of  the  great  number  of  agencies  at  work 
the  secondary  products  that  result  may  be  simpler  or  more 
complex,  according  to  conditions.  Some  bacteria  have  a  tend- 
ency, while  tearing  down  organic  matter,  to  produce  syn- 
thetic compounds,  which  present  a  very  complicated  molecule 
until  they  are  in  turn  degraded.  The  tendency  for  the  sec- 
ondary products  to  react  both  among  themselves  and  with  the 

1  Decomposition  and  decay  are  general  terms  referring  to  all  of  the 
degradation  processes  through  which  the  original  tissue  passes  in  the 
soil.  Fermentation  refers  to  the  decomposition  of  carbohydrates  while 
putrefaction  has  to  do  usually  with  nitrogenous  materials. 

2  A  catalytic  agent  is  a  material  capable  of  hastening  or  retarding  a 
chemical  reaction,  the  catalyst  emerging  unchanged  from  the  transforma- 
tion. Enzymes  are  catalysts  produced  by  living  organisms  and  may  be 
active  within  or  without  the  cell.    They  are  generally  colloidal  in  nature. 


104         NATURE  AND  PROPERTIES  OF  SOILS 


mineral  constituents  is  by  no  means  an  unimportant  factor 
in  accounting  for  the  complexity  of  the  decaying  organic  mat- 
ter. 

As  the  processes  of  fermentation  and  putrefaction  go  on 
the  complex  intermediate  compounds  are  gradually  broken 
down  and  certain  simple  products  result.  Such  materials  may 
result  from  a  progressive  simplification  of  the  partially  de- 

H1GHER 
PLANTS 


NUTRIENTS 


PLANT 
TISSUE 


LOST  FROM 
THE 
SOIL 


Fig.  21. — Diagram  showing  the  transformations  through  which  the  con- 
stituents of  the  plant  tissue  pass  from  the  time  the  organic  matter 
enters  the  soil  until  it  is  in  a  condition  to  be  used  by  succeeding 
crops.     The  cycle  is  very  largely  biological. 

cayed  matter  or  may  be  by-products  or  split-off  compounds 
from  the  more  complex  reactions.  These  simple  materials  are 
partially  solid  and  partially  gaseous.  Carbon  dioxide  is  a 
universal  product  of  bacterial  activity  of  all  kinds  and  is 
constantly  being  evolved.  Other  simple  constituents  arising 
from  organic  decay  are  water,  ammonia,  nitrites,  nitrates,  free 
nitrogen,  and  sulfur  dioxide.  Some  of  these  are  lost  from 
the  soil,  some  lose  their  identity  by  reacting  with  the  soil  con- 
stituents, while  others  may  function  as  plant  nutrients.    When 


THE  ORGANIC  MATTER  OF  THE  SOIL         105 

they  are  absorbed  again  by  a  crop,  the  organic  cycle  is  com- 
pleted. 

56.  The  partially  decomposed  organic  matter.1 — The 
most  complicated  parts  of  the  organic  matter  in  the  soil  are 
the  primary  and  secondary  products  of  decomposition,  the 
materials  between  the  original  tissue  and  the  simple  products. 
These  compounds  are  not  only  complex  but  they  are  contin- 
ually changing.  A  certain  compound  present  in  the  soil  one 
week  may  be  altered  the  next.  Again,  at  least  a  part  of  the 
decomposing  organic  matter  is  colloidal,  thus  possessing  spe- 
cial absorptive  and  catalytic  properties.  When  the  soil 
organic  matter  is  treated  with  the  various  extractive  agents, 
reactions  may  be  induced  which  would  not  take  place  in  a 
normal  soil.  Compounds  are  then  formed  which  would  prob- 
ably not  exist  under  natural  conditions. 

Many  chemists  have  worked  on  the  problems  of  the  con- 
stitution of  the  organic  matter  of  the  soil  and  have  published 
their  results.  The  early  conceptions  were  rather  simple. 
Mulder,2  for  example,  considered  the  soil  organic  matter  to 
consist  almost  entirely  of  carbon,  hydrogen,  and  oxygen.  Such 
a  concept  ignores  the  presence  of  nitrogen,  sulfur,  and  the 
mineral  elements  of  the  original  plant  tissue,  and  is  much 
too  simple  to  explain  organic  transformations. 

Even  the  investigators  3  of  Mulder 's  time  obtained  diseor- 

*See  Morrow,  C.  A.,  The  Organic  Matter  of  the  Soil;  A  Study  of  the 
Nitrogen  Distribution  in  Different  Soil  Types;  Dissertation,  Univ. 
Minn.,  1918. 

3  Mulder,  T.  J.,  Die  Organischen  Bestandtheile  im  Boden;  Chemie  der 
Ackerkrume,  I,  pp.  308-360,  Berlin,  1863.  Also,  Wiley,  H.  W.,  Agricul- 
tural Analysis;  Vol.  I,  p.  53,  Easton,  Pa.,  1906. 

Mulder  contended  that  the  organic  matter  consisted  of  seven  distinct 
compounds,  as  follows :  1  &  2,  Ulmic  acid  and  ulmin ;  3  &  4,  Humic  acid 
and  humin;  5,  Geic  acid;  6,  Apocrenic  acid;  7,  Crenic  acid.  These 
bodies  he  considered  as  arising  from  one  another  by  oxidation;  thus 
ulmic  acid  (C40H14O12)  gave  humic  acid  (C^H^O^),  which  in  turn  yielded 
geic  acid  (C^H^O^),  followed  by  apocrenic  acid  (C^B.^0^) ,  and  finally 
by  crenic  acid  (C24H12016). 

8  See  Schreiner,  O.,  and  Shorey,  E.  C,  The  Isolation  of  Harmful  Or- 
ganic Substances  from  Soils;  U.  S.  Dept.  Agr.,  Bur.  Soils,  Bui.  53,  pp. 
15-16,  1909. 


106         NATURE  AND  PROPERTIES  OF  SOILS 

dant  results,  but  these  were  explained  for  the  time  being  by 
assuming  that  the  discrepancies  occurred  because  of  added 
molecules  of  water. 

Later  investigators,  while  progressing  rather  slowly  toward 
definite  results,  did  accomplish  one  thing  of  importance.  They 
threw  considerable  doubt  on  the  old  ideas  of  the  Mulder 
school  of  chemists. 

One  of  the  men,  whose  work  established  beyond  a  doubt  the 
fact  that  organic  matter  was  a  mixture  of  very  complicated 
compounds,  was  Van  Bemmelen.1  His  investigations  still 
further  showed  that  the  soil  organic  matter  was  largely  in  a 
colloidal  condition,  and,  therefore,  exhibited  properties  quite 
distinct  from  those  shown  by  true  solutions  or  matter  in  a 
coarse  state  of  division. 

In  recent  years,  Baumann  2  by  his  researches  has  shown 
freshly  precipitated  organic  matter  to  possess  properties  which 
are  largely  colloidal  in  nature.  Among  these  characteristics 
are  high  water  capacity,  great  absorptive  power  for  certain 
salts,  ready  mixture  with  other  colloids,  power  to  decompose 
salts,  great  shrinkage  on  drying,  and  coagulation  in  the  pres- 
ence of  electrolytes.  Jodidi 3  has  studied  the  composition  of 
the  acid-soluble  organic  nitrogen  in  peat  and  mineral  soils. 
The  nitrogenous  compounds  thus  obtained  can  be  divided  into 
the  following  groups:  (1)  ammoniacal  nitrogen,  (2)  nitric 
nitrogen,  (3)  acids  amides,  (4)  mon-  and  diamino-acids.  The 
two  latter  groups 4  carry  the  bulk  of  the  organic  nitrogen, 

1  Van  Bemmelen,  J.  M.,  Die  Absorptions  Verbindungen  und  das  Ab- 
sorptionsvermogen  der  Ackererde;  Landw.  Versuch.  Stat.,  Band  35, 
Seite  67-136,  1888. 

2  Baumann,  A.,  Untersuchungen  tfber  die  Hummussauren ;  Mitt.  d.  K. 
bayr.  Moorkulturanstalt,  Heft  3,  Seite  53-123,  1909. 

"Jodidi,  S.  L.,  Organic  Nitrogenous  Compounds  in  Peat  Soils  I;  Mich. 
Agr.  Exp.  Sta.,  Tech.  Bui.  4,  Nov.,  1909.  Also,  The  Chemical  Nature  of 
the  Organic  Nitrogen  in  Soil;  la.  Agr.  Exp.  Sta.,  Ees.  Bui.  I,  June  1911. 

4  Amides  or  acid  amides  are  formed  from  organic  acids  by  replacing 
the  hydroxyl  of  the  carboxyl  group  with  NH2.    Acetic  acid  (CH3COOH) 


THE  ORGANIC  MATTER  OF  THE  SOIL         107 

but  quantitative  determinations  are  uncertain.  These  com- 
pounds produce  ammonia  readily,  the  rate  depending  on  their 
chemical  structure. 

The  present  knowledge  of  the  chemical  constitution  of  the 
soil  organic  matter  is  due  largely  to  investigations  prosecuted 
by  the  United  States  Bureau  of  Soils.1  As  a  result  of  several 
years  work  a  large  number  of  compounds  were  isolated.  Some 
are  original  constituents  of  the  plant  tissue  but  the  bulk  has 
arisen  through  the  process  of  organic  decomposition. 

The  compounds  isolated  were  classified  from  the  chemical 
standpoint  under  four  heads,  those  containing:  (1)  carbon 
and  hydrogen;  (2)  carbon,  hydrogen,  and  oxygen;  (3)  car- 
bon, hydrogen,  and  nitrogen,  or  carbon,  hydrogen,  oxygen, 
and  nitrogen;  (4)  sulfur  in  combination  with  any  or  all  of 
the  elements  listed  above.  With  the  possible  presence  in 
soils  of  compounds  containing  so  many  elements,  it  is  little 
wonder  that  the  subject  is  a  complicated  one.  It  is  evident, 
moreover,  that  any  list  now  available  will  be  only  partial,  and 
that  many  other  compounds  of  even  more  intricate  composi- 
tion will  be  isolated  later. 

A  list  of  some  of  the  compounds  isolated  from  soil  organic 
matter  by  the  Bureau  of  Soils  follows: 

thus  becomes  acet-amide  (CHaC0NH2).  Amino-acids  are  produced  by 
replacing  one  of  the  alkyl/hydrogens  with  NH2.  Acetic  acid  thereby 
becomes  amino-acetic  acid  or  glycocoll  (CH2(NH2)COOH).  Protein/hy- 
drolysis is  probably  as  follows: 

^Acid  amides 
Proteins  — >  Proteoses  — >  Peptones  — »  Peptides  S~ 

^Amino-acids 

1  Sehreiner,  O.  and  Shorey,  E.  C,  The  Isolation  of  Harmful  Substances 
from  Soils;  U.  S.  Dept.  Agr.,  Bur.  Soils,  Bui.  53,  1909;  also  Buls.  47,  70, 
74,  77,  80,  83,  87,  88,  and  90.  See  also,  Sullivan,  M.  X.,  Origin  of 
Vanillin  in  Soil;  Jour.  Ind.  &  Eng.  Chem.,  Vol.  6,  No.  11,  pp.  919-921, 
1914.  Kelley,  W.  P.,  The  Organic  Nitrogen  of  Hawaiian  Soils;  Jour. 
Amer.  Chem.  Soc,  Vol.  XXXVI,  No.  2,  pp.  429-444,  Feb.,  1914.  Walters, 
E.  H.,  Proteoses  and  Peptones  in  Soils;  Jour.  Ind.  &  Eng.  Chem.,  Vol. 
7,  No.  10,  pp.  860-863,  1915.  Lathrop,  E.  C,  Protein  Decomposition 
in  Soils;  Soil  Sci.,  Vol.  I,  No.  6,  pp.  509-532,  June,  1916. 


108         NATURE  AND  PROPERTIES  OF  SOILS 


Hentriacontane — C3JH64  Histidine — C6H902N3 

*>/  Dihydroxystearic  acid —  Trithiolbenzaldehyde — 


C18H3604  (C.H.CSH),. 

Succinic  acid — C4H604  Creatinine — C4H7ON3 

Picoline  carboxylic  acid —  Salicylic  Aldehyde — 

C7H702N  C6H4OHCOH 

57.  Relation  of  organic  compounds  to  plants. — So  far  as 
the  plant  is  concerned,  organic  compounds  may  be  divided 
into  three  groups:  those  that  are  beneficial,  those  that  are 
neutral,  and  those  that  are  toxic  or  harmful  in  their  effects. 
As  an  example  of  the  first  group,  histidine  and  creatinine  x 
may  be  mentioned.  Here  is  a  case  in  which  the  compounds 
in  the  soil  organic  matter  may  exert  a  stimulating  effect  on 
plant  growth,  supplementing  the  nitrates 2  to  a  certain  extent. 
That  the  nitrogen  of  the  soil  organic  matter  may  be  utilized 
by  plants  is  well  summarized  by  the  publications  of  Hutchin- 
son and  Miller.3  As  an  example  of  a  harmful  compound  aris- 
ing from  the  decomposition  of  the  organic  matter,  dihydroxy- 
stearic acid  may  be  mentioned  as  one  of  the  best  known.  This 
compound  was  the  first  to  be  isolated  and  identified  by  the 
Bureau  of  Soils  and  is  very  toxic. 

The  discovery  of  such  compounds  in  the  soil  has  revived  the 
old  theory  of  toxicity,4  by  which  the  infertility  of  certain 
soils  was  accounted  for.  Root  excretions  were  also  held  to  be 
detrimental  to  succeeding  crops  of  the  same  kind.  The  toxic 
materials  of  the  soil  organic  matter  largely  originate  under 

1  Skinner,  J.  J.,  Effect  of  Histidine  and  Arginine  as  Soil  Constituents ; 
Eighth  Internat.  Cong.  App.  Chem.,  Vol.  XV,  pp.  253-264,  1912.  Also, 
Beneficial  Effects  of  Creatinine  and  Creatine  on  Growth;  Bot.  Gaz., 
Vol.  54,  No.  2,  pp.  152-163,  1912. 

2Schreiner,  O.,  and  Skinner,  J.  J.,  Nitrogenous  Soil  Constituents  and 
Their  Bearing  upon  Soil  Fertility;  U.  S.  Dept.  Agr.,  Bur.  Soils,  Bui.  87, 
p.  68,  1912.  Also,  Schreiner,  O.,  and  Others,  A  Beneficial  Organic  Con- 
stituent of  Soils;  Creatinine;  U.  S.  Dept.  Agr.,  Bur.  Soils,  Bui.  83,  p.  44, 
1911. 

8  Hutchinson,  H.  B.,  and  Miller,  N.  H.  J.,  The  Direct  Assimilation 
of  Inorganic  and  Organic  Forms  of  Nitrogen  by  Higher  Plants;  Jour. 
Agr.  Sci.,  Vol.  4,  Part  3,  pp.  282-302,  1912. 

*See  Schreiner,  O.,  and  Reed,  H.  S.,  Some  Factors  Influencing  Soil 
Fertility;  U.  S.  Dept.  Agr.,  Bur.  Soils,  Bui.  40,  pp.  36-40,  1907. 


THE  ORGANIC  MATTER  OF  THE  SOIL         109 

conditions  of  poor  drainage  and  aeration.  The  toxicity  of 
such  compounds  as  dihydroxystearic  acid,  picoline  carboxylic 
acid  and  aldehydes  may,  therefore,  be  overcome  by  oxidation.1 
Good  soil  aeration  is  a  factor  in  dealing  with  such  conditions. 

Fertilizers,  according  to  Schreiner  and  Skinner,2  seem  to 
decrease  the  harmful  effects  of  such  compounds;  nitrogenous 
fertilizers  overcoming  some  toxic  materials,  and  phosphoric 
acid  or  potash  neutralizing  others.  Robbins  3  has  shown  that 
soil  organisms  have  the  power  of  causing  the  disappearance 
of  certain  toxic  materials  in  the  soil,  such  as  cumarin,  vanillin, 
pyridine,  and  quinoline. 

While  Schreiner  found  twenty  soils,  out  of  a  group  of  sixty 
taken  in  eleven  states  of  this  country,  to  contain  dihydroxy- 
stearic acid,  this  does  not  necessarily  mean  that  this  or  sim- 
ilar compounds  are  serious  detrimental  factors.  It  is  very 
likely  that  such  compounds  are  merely  products  of  improper 
soil  conditions,  and  are  to  be  considered  as  concomitant  with 
depressed  crop  yield.  When  such  conditions  are  righted,  the 
so-called  toxic  matter  will  disappear,  as  has  been  shown  by 
the  researches  of  Davidson.4  Good  drainage,  lime,  tillage, 
aeration,  and  oxidation,  are  so  efficacious  in  this  regard  that 
permanent  organic  soil  toxicity  need  never  be  a  factor  in  soils 
rationally  managed. 

1  Schreiner,  O.,  and  Others,  Certain  Organic  Constituents  of  Soils  in 
Eelation  to  Soil  Fertility;  U.  S.  Dept.  Agr.,  Bur.  Soils,  Bui.  47,  p.  52, 
1907.  Also,  Schreiner,  O.,  and  Reed,  H.  S.,  The  Bole  of  Oxidation  in 
Soil  Fertility;  U.  S.  Dept.  Agr.,  Bur.  Soils,  Bui.  56,  p.  52,  1906. 

2  Schreiner,  O.,  and  Skinner,  J.  J.,  Organic  Compounds  and  Fertilizer 
Action;  U.  S.  Dept.  Agr.,  Bur.  Soils,  Bui.  77,  1911.  Also,  Experi- 
mental Study  of  the  Effect  of  Some  of  the  Nitrogenous  Soil  Constituents 
on  Growth;  Plant  World,  Vol.  16,  No.  2,  pp.  45-60,  Feb.,  1913. 

3  Robbins,  W.  J.,  The  Cause  of  the  Disappearance  of  Cumarin,  Vanillin, 
Pyridine  and  Quinoline  in  the  Soil;  Ala.  Agr.  Exp.  Sta.,  Bui.  195,  June, 
1917.  Also,  The  Destruction  of  Vanillin  in  the  Soil  by  the  Action  of 
Soil  Bacteria;  Ala.  Agr.  Exp.  Sta.,  Bui.  204,  June,  1918.  Robbins,  W.  J., 
and  Massey,  A.  B.,  The  Effect  of  Certain  Environmental  Conditions  on 
the  Bate  of  Destruction  of  Vanillin  by  a  Soil  Bacterium;  Soil  Sci., 
Vol.  X,  No.  3,  pp.  237-246,  Sept.,  1920. 

4  Davidson,  J.,  A  Comparative  Study  of  the  Effects  of  Cumarin  and 
Vanillin  on  Wheat  Grown  in  Soil,  Sand  and  Water  Culture;  Jour. 
Amer.  Soc.  Agron.,  Vol.  7,  No.  4,  pp.  145-158,  1915. 


110         NATURE  AND  PROPERTIES  OF  SOILS 


58.    Simple  products  of  organic  decomposition.— As  the 

processes  of  chemical  and  biological  change  of  the  soil  organic 
matter  proceed,  the  simple  compounds  already  noted  begin 
to  appear.  This  change  is  of  course  coordinate  with  a  certain 
amount  of  synthetic  action,  but  compounds  thus  built  up 
must  ultimately  succumb  to  the  agencies  at  work  and  suffer  a 
splitting-up  and  reduction  to  simple  bodies.  Carbon  dioxide 
is  one  of  the  most  important  of  these  compounds,  always  being 
a  product  of  bacterial  activity.  Its  importance  has  already 
been  noted  in  the  discussion  of  weathering.  Here  it  heightens 
the  solvent  power  of  water  and  tends  to  increase  the  amount  of 
nutrient  material  carried  in  the  soil  solution.  Carbonation 
is  a  direct  result  of  its  presence. 

With  increased  organic  matter  in  any  soil,  greater  bacterial 
action  and  an  increase  in  the  carbon  dioxide  evolved  may  well 
be  expected.  In  fact,  the  carbon  dioxide  production  of  a 
soil  is  considered  by  some  authors *  to  be  a  measure  of  bacterial 
activity.  With  this  increase  in  carbon  dioxide,  the  soil  air 
is  markedly  reduced  in  its  free  oxygen  and  an  alteration  in 
bacterial  and  plant  relationships  may  thereby  be  induced. 
The  following  figures  by  Wollny  2  show  the  composition  of 
the  soil  atmosphere  and  the  effects  of  additional  organic  ma- 
terial on  the  carbon  dioxide  content :    f 

Table  XXI 


Soils 

Percentage  by  "Volume  of 

C02 

0 

Atmospheric  air 

*      .04 
2.54 
1.06 
9.74 

20.96 

Soil  air  (average  19  analyses) 

A  sandy  soil 

18.33 
19.72 

Sandy  soil  plus  manure 

10.35 

1Stoklasa,  J.,  and  Ernest,  A.,  Pber  den  TJrsprung,  die  Menge,  und  die 
Bedeutung  des  Kohlendioxyds  im  Boden;  Centrlb.  Bakt.,  II,  14,  Seite 
723-736,  1905. 

aWoUny,  E.,  Die  Zersetzung  der  Organischen  Stoffe;  Seite  2,  Heidel- 
berg, 1897. 


THE  ORGANIC  MATTER  OF  THE  SOIL         111 

While  carbon  dioxide  may  be  evolved  by  the  splitting-up 
of  both  carbohydrate  and  nitrogenous  bodies,  ammonia  re- 
sults only  from  the  latter.  It  is  really  the  first  extremely 
simple  nitrogenous  body  produced.  It  can  be  utilized  by 
some  plants  as  a  source  of  nitrogen,  as  is  also  true  of  certain 
products  of  partial  decomposition  such  as  urea,  but  ordinarily 
it  must  undergo  oxidation.  This  oxidation  results  in  nitrites 
(N02)  and  ultimately  in  nitrates  (N03),  the  latter  usually 
being  considered  as  the  chief  source  of  the  nitrogen  utilized 
by  plants. 

Other  simple  products,  such  as  methane  (CH4),  hydrogen 
disulphide  (H2S),  carbon  disulphide  (CS2),  and  the  like,  may 
also  result.  They  are  relatively  unimportant,  however,  as 
regards  the  plant,  in  comparison  with  the  role  played  by  car- 
bon dioxide,  ammonia,  the  nitrites,  and  the  nitrates.  The 
production  of  the  nitrates  from  ammonia  is  very  closely  cor- 
related with  good  soil  conditions,  especially  optimum  moisture 
and  adequate  aeration.  The  proper  handling  of  the  soil,  then, 
will  not  only  tend  to  eliminate  toxic  matter  and  prevent  its 
further  formation  but  will  encourage  the  proper  decay  of  the 
soil  organic  matter  and  the  production  of  simple  compounds 
wThich  will  function  directly  or  indirectly  as  nutrients. 

59.  Carbonized  materials  of  soil. — After  the  extraction 
of  the  soil  for  the  study  of  the  ordinary  organic  compounds, 
a  considerable  mass  of  material  remains,  which  is  insoluble 
in  water,  alkali,  and  other  ordinary  solvents.  By  the  extrac- 
tion of  a  large  amount  of  soil,  Schreiner  and  Brown  x  were 
able  to  study  this  material.  They  found  it  susceptible  to  *di- 
vision  into  six  groups,  as  follows:  (1)  plant  tissue,  (2)  insect 
and  other  organized  material,  (3)  charcoal  particles,  (4)  lig- 
nite, (5)  coal  particles,  and  (6)  materials  resembling  natural 
hydrocarbons,  as  bitumen,  asphalt,  and  the  like.     Such  ma- 

1  Schreiner,  O.,  and  Brown,  B.  E.,  Occurrence  and  Nature  of  Carbon- 
ized Material  in  Soils;  U.  S.  Dept.  Agr.,  Bur.  Soils,  Bui.  90,  1912. 


112        NATURE  AND  PROPERTIES  OF  SOILS 

terial  was  found  not  only  near  the  surface  of  the  soil  but  at 
depths  of  fifteen  or  twenty  feet. 

The  exact  origin  of  this  material  is  problematical.  Forest 
and  prairie  fires,  infiltration,  mild  oxidation,  and  lignifica- 
tion  might  be  mentioned.  Of  a  certainty  the  agencies  of  dis- 
tribution are  the  natural  forces  engaged  in  physical  weather- 
ing. Such  material  can  be  divided. into  two  general  groups, 
organized  and  unorganized;  in  the  former,  the  normal  struc- 
ture remains  intact,  while  in  the  latter  the  original  features 
have  been  obliterated.  Part  of  it  belongs,  therefore,  in  the 
original  plant  tissue  group ;  a  part  of  it  with  the  partially  de- 
cayed material;  while  some  must  be  included  with  the  simple 
products  of  decomposition.  This  carbonized  material  is  im- 
portant, as  it  makes  up  no  inconsiderable  part  of  the  soil 
organic  matter.  It  is  very  resistant,  and  consequently  lends 
stability  to  the  organic  constituents. 

60.  The  determination  of  soil  organic  matter.1 — A  num- 
ber of  methods  have  been  proposed  for  the  direct  or  indirect 
determination  of  the  organic  matter  in  soils,  but  none  has 
proved  entirely  satisfactory,  since  the  composition  of  this  ma- 
terial is  so  indefinite  and  complicated  and  so  likely  to  change 
while  under  investigation.  Other  soil  constituents  also  tend 
to  interfere  with  the  determination.  Three  general  methods 
seem  worthy  of  mention,  as  they  have  been  used  very  widely 
in  soil  analyses  and  at  least  give  comparative,  if  not  absolutely 
accurate,  results.  They  will  be  discussed  in  the  inverse  order 
of  their  value. 

Loss  of  ignition.2 — This  is  a  simple  method  which  designs 
to  burn  off  the  organic  matter  and  determine  its  loss  by  dif- 
ference. Five  grams  of  dry  soil  are  placed  in  a  crucible  and 
ignited  at  a  low  red  heat  until  the  organic  matter  is  all  oxi- 

1Soil  organic  matter  as  here  used  refers  only  to  the  original  and 
partially  decayed  organic  constituents.  Carbon  dioxide,  methane,  nitrites, 
nitrates  and  similar  compounds  are,  therefore,  not  included  in  this  term. 

3  Wiley,  H.  W.,  Official  and  Provisional  Methods  of  Analysis;  U.  S. 
Dept.  Agr.,  Bur.  Chem.,  Bui.  107,  p.  19,  1908. 


THE  ORGANIC  MATTER  OF  THE  SOIL         113 

dized.  The  cold  mass  is  moistened  with  ammonium  carbonate 
and  heated  to  a  temperature  of  150°  C.  in  order  to  expel  the 
excess  of  ammonia  and  replace  the  carbon  dioxide.  The 
change  in  weight  is  rated  as  loss  on  ignition. 

This  method  is  open  to  the  objection  that,  besides  the  loss 
of  organic  matter,  a  certain  amount  of  water  of  combina- 
tion, and  all  ammoniacal  compounds,  nitrates,  carbon  dioxide, 
and  some  alkali  chlorides,  if  the  temperature  is  carried  too 
high,  are  driven  off.  The  method,  therefore,  gives  high  results, 
especially  in  the  presence  of  large  amounts  of  hydrated  sili- 
cates such  as  are  likely  to  occur  in  residual  soils.  Notwith- 
standing these  objections,  this  method  has  been  used  to  a  very 
great  extent  in  soil  analysis.1 

Chromic  odd  method. — This  method,  proposed  by  Wolff, 
has  been  modified  and  improved  by  various  chemists.  War- 
ington  and  Peake  2  have  perhaps  done  more  with  the  method 
than  any  other  investigators.  In  the  United  States  the  modi- 
fication by  Cameron  and  Breazeale  3  has  been  very  generally 
accepted.4  It  consists  in  the  treatment  of  the  soil  sample  with 
sulfuric  acid,  and  chromic  acid,  or  potassium  bichromate. 
The  organic  matter,  in  the  presence  of  the  sulfuric  acid  and 
an  oxidizing  agent,  evolves  carbon  dioxide  until,  if  the  mix- 

1  Eather  offers  a  modification  to  this  method  which  seems  to  obviate 
some  of  its  difficulties.  The  soil  is  first  extracted  with  dilute  HC1  and 
HF  to  remove  the  hydrated  aluminum  silicates,  the  organic  matter  being 
little  influenced  thereby.  The  sample  is  then  ignited  in  the  usual 
manner.  Bather,  J.  B.,  An  Accurate  Loss-on-Ignition  Method  for  the 
Determination  of  Organic  Matter  in  Soils;  Jour.  Ind.  and  Eng.  Chem., 
Vol.  X,  No.  6,  pp.  439-442,  June,  1918. 

2  Warington,  B.,  and  Peake,  W.  A.,  On  the  Determination  of  Carbon  in 
Soils;  Jour.  Chem.  Soc.  (London),  Trans.,  Vol.  37,  pp.  617-625,  1880. 

3Briggs,  L.  J.,  and  others,  The  Centrifugal  Methods  of  Mechanical 
Soil  Analysis;  U.  S.  Dept.  Agr.,  Bur.  Soils,  Bui.  24,  pp.  33-38,  1904. 
Also,  Cameron,  F.  K.,  and  BTeazeale,  J.  F.,  The  Organic  Matter  in  Soils 
and  Subsoils;  Jour.  Amer.  Chem.  Soc,  Vol.  26,  pp.  29-45,  1904. 

4Waynick  offers  a  simplification  of  this  method:  Waynick,  D.  D.,  A 
Simplified  Wet  Combustion  Method  for  the  Determination  of  Carbon  in, 
Soils;  Jour.  Ind.  and  Eng.  Chem.,  Vol.  XI,  No.  7,  pp.  634-637,  1919. 


114        NATURE  AND  PROPERTIES  OF  SOILS 

ture  is  boiled,  practically  all  of  the  carbon  is  thus  driven  off. 
This  gas  is  drawn  through  a  train  of  absorption  bulbs,  caught 
in  a  solution  of  potassium  hydroxide,  and  thus  weighed. 

A  second  determination  is  now  made  on  a  new  sample  of 
soil,  leaving  out  the  chromic  acid.  The  carbon  dioxide  given 
off  under  such  conditions  is  that  of  an  inorganic  nature.  The 
weight  of  this  gas  substracted  from  the  total  carbon  dioxide 
leaves  the  organic  carbon  dioxide. 

The  data  from  the  use  of  the  chromic  acid  method  may  be 
expressed  as  organic  carbon  or  as  organic  matter.  Multiply- 
ing the  carbon  dioxide  by  .471  or  the  carbon  by  1.724  is  con- 
sidered as  giving  an  approximate  figure  for  the  organic  mat- 
ter. 

The  results  obtained  with  the  chromic  acid  method  are  usu- 
ally lower  than  those  from  ignition  or  combustion,  due  par- 
tially to  the  oxidation  resistance  of  the  carbonized  matter, 
already  discussed.  This  material,  while  it  succumbs  to  igni- 
tion, resists  the  action  of  the  sulfuric  and  chromic  acids  to 
a  very  large  degree.  The  water  of  hydration  is,  of  course,  not 
a  factor  in  the  chromic  acid  method. 

Bomb  Combustion} — Two  grams  of  soil,  .75  gram  of  mag- 
nesium powder,  and  10  grams  of  sodium  peroxide  (Na202) 
are  thoroughly  mixed  in  a  closed  dry  calorimeter  bomb.  The 
mixture  is  then  exploded  by  heating,  all  of  the  carbon  of  the 
soil  being  changed  to  the  carbonate  form  by  the  reaction. 
The  fused  charge  is  now  removed  to  a  flask  and  by  treating 
with  acid,  the  carbon  in  the  form  of  carbon  dioxide  may  be 
driven  off  into  a  Parr  apparatus  and  measured  under  stand- 
ard conditions  of  temperature  and  pressure. 

The  amount  of  inorganic  carbonate  carbon  in  the  soil  must 

1  Wiley,  H.  W.,  Official  and  Provisional  Methods  of  Analysis;  U.  S. 
Dept.  Agr.,  Bur.  Chem.,  Bui.  107,  p.  234,  1908. 

There  are  a  number  of  other  methods  of  complete  combustion.  Very 
often  the  combustion  is  carried  on  in  a  current  of  oxygen  over  hot 
cuprous  oxide.  The  organic  carbon  may  thus  be  very  accurately 
determined. 


THE  ORGANIC  MATTER  OF  THE  SOIL         115 


be  determined  on  a  separate  sample  and  deducted  from  the 
figure  obtained  by  the  combustion  above  described.  This  will 
give  the  organic  carbon  of  the  soil  in  terms  of  carbon  dioxide. 
The  percentage  of  organic  carbon  may  now  be  calculated  as 
well  as  the  approximate  amount  of  organic  matter  (C  X  1.724 
s=  organic  matter  or  C02  X  .471  =  organic  matter.)1 

61.  Determination  of  soil  humus. — Humus 2  is  a  term  ap- 
plied to  that  portion  of  the  organic  matter  which  can  be  re- 
moved with  ammonium  hydroxide  after  the  soil  has  been 
treated  with  hydrochloric  acid  and  washed  free  thereof.  The 
common  method  of  humus  estimation  is  that  proposed  by 
Grandeau.3  The  sample  of  soil  is  first  washed  with  acid  in 
order  to  remove  the  bases  in  combination  with  the  organic  mat- 
ter. It  is  next  treated  with  ammonia,  which  will  then  dissolve 
out  the  humous  materials.  The  method  is  based  on  the  fact 
that  when  a  soil  is  lacking  in  active  basic  material,  certain 
parts  of  the  organic  matter  are  soluble  in  an  alkali.  The  dark 
humous  extract  obtained  with  the  ammonia  is  called  Matiere 
Noire  and  is  supposed  to  be  the  most  active  part  of  the  soil 
organic  matter. 

This  method  has  undergone  several  modifications  4  of  which 

1  Wiley  presents  the  following  comparisons  of  the  three  methods  dis- 
cussed above: 


Soil 

Ignition 

Combustion 
(c  x  1.724) 

Chromic  acid 
(c  x  1.724) 

Old  pasture  

9.27 
7.07 
5.95 

6.12 
4.16 
2.44 

4  84 

New  pasture 

3  32 

Arable  soil   

2.03 

Wiley,  H.  W.,  Principles  and  Practices  of  Agricultural  Analysis,  Vol. 
1,  pp.  352-354,  Easton,  Pa.,  1906. 

2 The  term  " humus"  is  used  in  a  number  of  different  ways.  Conti- 
nental Europeans  make  it  synonymous  with  organic  matter.  In  some  cases 
it  is  used  to  indicate  all  of  the  partially  decayed  material  of  the  soil.  The 
restricted  meaning  employed  in  this  text  is  less  confusing  as  it  coincides 
with  the  chemical  interpretation.  Grandeau  believed  the  organic  matter 
thus  dissolved  was  a  determining  factor  in  soil  fertility. 

3  Grandeau,  L.,  Traiti  d' Analyse  de  Matieres  Agricoles;  I,  p.  151,  1897. 

4 A  comparison  of  the  various  methods  is  found  as  follows:     Alway, 


116         NATURE  AND  PROPERTIES  OF  SOILS 

that  of  Hilgard  1  and  that  of  Houston  and  McBride  2  seem 
most  important. 

In  the  procedure  an  attempt  is  made  to  keep  the  concen- 
tration of  the  ammonia  in  contact  with  the  soil  constant  dur- 
ing the  extraction.  Consequently  the  sample,  after  treatment 
with  the  acid,  is  washed  into  a  500  cubic  centimeter  flask, 
which  is  filled  to  the  mark  with  4  per  cent,  ammonia.  Diges- 
tion is  allowed  to  proceed  for  twenty-four  hours,  with  fre- 
quent shakings.  The  solution  is  then  filtered  and  evaporated 
to  dryness.  The  residue  is  weighed,  after  drying  thoroughly 
at  100°  C,  and  then  ignited,  the  loss  being  considered  as 
humus. 

This  method  is  open  to  serious  criticism  in  that  it  is  wholly 
arbitrary  and  subject  to  considerable  inaccuracy  through 
manipulation  and  the  ignition  of  the  humic  residue.  There 
is  also  some  doubt  whether  the  figures  obtained  have  any 
direct  relation  to  the  fertility  of  the  soil.3        ^ 

62.  The  organic  matter  and  nitrogen  of  representative 
soils. — The  amount  of  organic  matter  in  soils  varies  so  widely 
according  to  the  nature  of  the  soil  and  climate  conditions  that 
it  is  difficult  to  present  representative  figures.  Excluding 
peat  and  muck,  which  are  20  to  80  per  cent,  organic,  the  aver- 
age mineral  surface  soil  is  found  to  contain  from  .50  per  cent, 
to  18  or  20  per  cent,  of  organic  matter.  Some  surface  soils 
of  West  Virginia,4  averaging  2.88  per  cent,  organic  matter, 

F.  J.,  and  others,  The  Determination  of  Humus;  Neb.  Agr.  Exp.  Sta., 
Bui.  115,  June,  1910. 

1  Hilgard,  E.  W.,  Humus  Determination  in  Soils;  U.  S.  Dept.  Agr., 
Div.  Chem.,  Bui.  38  (edited  by  H.  W.  Wiley),  p.  80,  1893. 

"Houston,  H.  A.,  and  McBride,  F.  W.,  A  Modification  of  Grandeau's 
Method  for  the  Determination  of  Humus;  U.  S.  Dept.  Agr.,  Div.  Chem., 
Bui.  38  (edited  by  H.  W.  Wiley),  pp.  84-92,  1893.  See  also,  Smith,  O.  C, 
A  Proposed  Modification  of  the  Official  Method  of  Determining  Humus; 
Jour.  Ind.  and  Eng.  Chem.,  Vol.  5,  No.  1,  pp.  35-37,  Jan.,  1913. 

•Gortner,  R.  A.,  The  Organic  Matter  of  the  Soil;  III.  On  the  Pro- 
duction of  Humus  from  Manures;  Soil  Sci.,  Vol.  Ill,  No.  1,  pp.  1-8,  Jan., 
1917.  Carr,  R.  H.,  Is  the  Humus  Content  of  the  Soil  a  Guide  to  Fer- 
tility;  Soil  Sci.,  Vol.  Ill,  No.  6,  pp.  515-524,  June,  1917. 

4  Salter,  R.  M.,  and  Wells,  C.  F.,  Analyses  of  West  Virginia  Soils; 
W.  Va.  Agr.  Exp.  Sta.,  Bui.  168,  Dec,  1918. 


'-» 


THE  ORGANIC  MATTER  OF  THE  SOIL         117 

range  from  .73  per  cent,  to  15.14  per  cent.,  while  similar  fig- 
ures on  the  Russian  Tschernozen  x  vary  from  3.45  to  16.72 
with  an  average  of  8.07  per  cent.  The  subsoil  of  course  runs 
lower  in  every  case.  The  following  figures,  while  far  from 
representative,  are  suggestive: 

Table  XXII 

PERCENTAGE     OF     ORGANIC     MATTER      (C  X  1.724)     IN     CERTAIN 
REPRESENTATIVE  SOILS  OP  THE    UNITED    STATES. 


Description 


2 

6 

30 


Residual  soils — Robinson  2 

Glacial  and  loessial  soils — Robin- 
son 2 

Kansas  till  soils — Call 3 

Nebraska  loess  soils — Alway  4.  . .  . 

Minnesota  till,  soils — Rost  and 
Alway  5 


Surface 


1.76 

4.59 
2.86 
3.83 

7.46 


Subsoil 


.64 

1.44 
1.98 
1.96 

1.88 


As  the  soil  nitrogen  is  carried  almost  wholly  by  the  organic 
matter,  and  is  a  true  organic  constituent  of  the  soil,  its  con- 
sideration at  this  point  is  opportune.  The  nitrogen  6  of  soils 
varies  with  the  organic  matter  and  may  range  in  surface 
mineral  soils  from  .01  to  .60  per  cent.    West  Virginia  7  soils, 

1  Kossowitsch,  P.,  Die  Schwarzerde ;  Internat.  Mitt.  f.  Bodenkunde, 
Band  I,  Heft  3-4,  S.  316,  1912. 

2  Robinson,  W.  O.,  The  Inorganic  Composition  of  Some  Important 
American  Soils;  U.  &  Dept.  Agr.,  Bui.  122,  1914. 

3  Call,  L.  E.,  et  al*,  Soil  Survey  of  Shawnee  County,  Kansas;  Kans. 
Agr.  Exp.  Sta.,  Bui.  200,  1914. 

4  Alway,  F.  J.,  and  McDole,  Gr.  R.,  The  Loess  Soils  of  the  Nebraska 
Portion  of  the  Transition  Begion:  I.  Hygroscopicity,  Nitrogen  and 
Organic  Carbon;  Soil  Sci.,  Vol.  I,  No.  3,  pp.  197-238,  Mar.,  1916. 

5  Rost.,  C.  O.,  and  Alway,  F.  J.,  Minnesota  Glacial  Soil  Studies;  I.  A 
Comparison  of  the  Soils  of  the  Late  Wisconsin  and  Iowan  Drifts; 
Soil  Sci.,  Vol.  XI,  No.  3,  pp.  161-200,  Mar.,  1921. 

6  Soil  nitrogen  is  determined  by  either  the  Kjeldahl  or  the  Gunning 
method.     These  will  be  described  later.    See  paragraph  165. 

7  Salter,  R.  M.,  and  Wells,  C.  F.,  Analyses  of  West  Virginia  Soils; 
W.  Va.  Agr.  Exp.  Sta.,  Bui.  168,  Dec,  1918. 


118         NATURE  AND  PROPERTIES  OF  SOILS 

for  example,  while  averaging  .147  per  cent,  nitrogen,  range 
from  .043  to  .539.  Louisiana  l  soils  average  .049  per  cent,  with 
a  range  from  .001  to  .109.  In  muck  and  peat  the  amount  of 
nitrogen  is  much  higher,  attaining  in  some  cases  3  per  cent. 

The  following  figures  indicate  the  nitrogen  contents  that 
may  be  expected  in  average  soils: 

Table  XXIII 

PERCENTAGE    OP    NITROGEN    IN    CERTAIN    REPRESENTATIVE    SOILS 
OP  THE  UNITED  STATES 


Description 

Soil 

Subsoil 

71  Cecil  soils  of  North  Carolina 2 

165  Norfolk  soils  of  North  Carolina 3. . 

16  Loess  soils  of  Central  U.  S.* 

381  Kentucky  soils  5 

.048 
.039 
.154 
.120 
.338 

.024 
.020 
.083 
.070 

30  Minnesota  till  soils  ■ 

.092 

While  the  ratio  between  the  respective  amounts  of  soil 
nitrogen  and  organic  matter  is  no  more  constant  than  that 
between  the  organic  carbon  and  the  organic  matter 
(C  X  1.724  =  organic  matter),  it  is  of  some  general  value.    If 

1  Walker,  S.  S.,  Chemical  Composition  of  Some  Louisiana  Soils  as  to 
Series  and  Texture;  La.  Agr.  Exp.  Sta.,  Bui.  177,  Aug.,  1920. 

-  Williams,  C.  B.,  et  al.,  Report  on  the  Piedmont  Soils,  Particularly 
with  Reference  to  their  Nature,  Plant-food  Requirements  and  Adapta- 
bility to  Different  Crops;  Bui.  N.  C.  Dept.  Agr.,  Vol.  36,  No.  2,  Feb., 
1915. 

3  Williams,  C.  B.,  et  al.,  Report  on  Coastal  Plain  Soils,  Particularly 
with  Reference  to  their  Nature,  Plant-food  Requirements  and  Suitability 
for  Different  Crops;  Bui.  N.  C.  Dept.  Agr.,  Vol.  39,  No.  5,  May,  1918. 

4  Robinson,  W.  O.,  et  al.,  Variation  in  tlie  Chemical  Composition  of 
Soils;  U.  S.  Dept.  Agr.,  Bui.  551,  June,  1917.  Alway,  F.  J.,  and  McDole, 
G.  R.,  The  Loess  Soils  of  the  Nebraska  Portion  of  the  Transition  Re- 
gion: I.  Hygroscopicity,  Nitrogen  and  Organic  Carbon;  Soil  Sci., 
Vol.  I,  No.  "5,  pp.  197-238,  Mar.,  1916.  Also,  Bennett,  H.  H.,  Soils  and 
Agriculture  of  the  Southern  States,  pp.  332-353 ;  New  York,  1921. 

6  Averitt,  S.  D.,  The  Soils  of  Kentucky;  Ky.  Agr.  Exp.  Sta.,  Bui.  193, 
July,  1915. 

8Rost,  C.  O.,  and  Alway,  F.  J.,  Minnesota  Glacial  Soil  Studies:  I.  A 
Comparison  of  the  Soils  of  the  Late  Wisconsin  and  the  Iowan  Drifts; 
Soil  Sci.,  Vol.  XI,  No.  3,  pp.  161-200,  Mar.,  1921. 


X 


THE  ORGANIC  MATTER  OF  THE  SOIL 


119 


the  percentage  of  nitrogen  in  the  soil  is  multiplied  by  20,  a 
rough  idea  of  the  amount  of  organic  matter  may  be  obtained 
(N  X  20  =  organic  matter).  The  following  data  from  Rost 
and  Alway x  illustrate  not  only  the  variations  in  organic 
matter  and  nitrogen  that  may  be  expected  in  the  surface  and 
subsurface  of  different  soils,  but  the  correlation  between  the 
organic  matter  and  nitrogen  just  mentioned: 

Table  XXIV 

AVERAGE    PERCENTAGE    OF    ORGANIC    MATTER     (C  X  1.724)     AND 

NITROGEN  IN  THIRTY  REPRESENTATIVE  MINNESOTA  TILL  SOILS 

FROM   THREE  SERIES.      THE  FIGURES  FOR  EACH   OF   THE 

THREE  SOIL  TYPES  ARE  AVERAGES  OF  TEN  ANALYSES. 


Depth 

Forest 

Carrington 

Loam 

Upland  Prairie 

Carrington  Silt 

Loam 

Lowland 

Prairie 

Fargo  Silt  Loam 

Organic 

Matter 

Nitro- 
gen 

Organic 

Matter 

Nitro- 
gen 

Organic 

Matter 

Nitro- 
gen 

1 —  6  inches.  . . 

7—12     " 

13—24    " 

25—36     " 

5.34 

2.41 

1.38 

.86 

.253 
.119 

.078 
.041 

7.96 
6.00 
3.11 
1.31 

.373 

.285 
.165 
.062 

13.08 
8.00 
3.24 
1.39 

.616 
.385 
.150 
.054 

The  following  tentative  classification  of  mineral  soils  on  the 
basis  of  their  percentages  of  organic  matter  and  nitrogen  is 
offered  for  generalized  field  use : 


Table  XXV 


Description 


Percentage  of 
Organic  Matter 


Percentage  op 
Nitrogen 


Low 

Medium  . 

High 

Very  high 


.0—  3.0 

3.0—  6.0 

6.0—10.0 

above  10.0 


.00—  .10 

.10—  .25 

.25—  .40 

above  .40 


1Eost,  C.  O.,  and  Alway,  F.  J.,  Minnesota  Glacial  Soil  Studies:  I.  A 
Comparison  of  the  Soils  of  the  Late  Wisconsin  and  the  lowan  Drifts; 
Soil  Sci.,  Vol.  XI,  No.  3,  pp.  161-200,  Mar.,  1921. 


120         NATURE  AND  PROPERTIES  OF  SOILS 

63.  The  humus  content  of  soils  is  of  course  lower  than 
the  organic  matter  contained  in  them.  It  likewise  varies 
according  to  climate  and  region,  not  only  in  amount,  but  also 
in  composition.  The  following  data  from  Hilgard *  and 
Alway 2  illustrate  these  points : 

Table  XXVI 

the  composition  of  california  arid  and  humid  soils, 
(hilgard) 


Description 

Humus  in 

Soil 
(Percentage) 

Nitrogen  in 
Humus 

(Percentage) 

Nitrogen  in 

Soil 
(Percentage) 

41  Arid  uplands  soils 

15  Subirrigated  arid  soils. . 
24  Humid  soils 

.91 
1.06 
4.58 

15.23 

8.38 
4.23 

.135 
.099 
166 

Table  XXVII 

COMPARATIVE  COMPOSITION  OF  SEMI-ARID  (WAUNETA)  AND  HUMID 
(WEEPING    WATER)    LOESS    SOILS    OF    NEBRASKA.       ( ALWAY ) 


Depth 

Organic  Matter 
(Percentage) 

Humus 
(Percentage) 

Nitrogen 
(Percentage) 

Wauneta 

Weeping 
Water 

Wauneta 

Weeping 
Water 

Wauneta 

Weeping 
Water 

1st  foot. 
2nd  ".. 

3rd  "... 
4th  "... 
5th  "... 
6th  "... 

2.77 

1.38 

1.09 

.79 

.55 

.45 

4.98 

.3.02 

1.38 

.83 

.45 

.36 

1.02 
.65 
.48 
.34 
.26 
.26 

2.34 
1.29 
.55 
.27 
.23 
.19 

.136 

.082 
.065 
.046 
.038 
.030 

.236 
.154 
.083 
.059 
.043 
.038 

1  Hilgard,  E.  W.,  Soils,  pp.  136-137;  New  York,  1911.  For  further 
data  regarding  Hilgard 's  conclusions  see:  Alway,  F.  J.,  and  Bishop, 
E.  S.,  Nitrogen  Content  of  the  Humus  of  Arid  Soils;  Jour.  Agr.  Ecs., 
Vol.  5,  No.  20,  pp.  909-916,  Feb.,  1916. 

2  Alway,  F.  J.,  et  al.,  The  Loess  Soils  of  the  Nebraska  Portion  of  the 
Transition  Eegion:  I.  Eygroscopicity,  Nitrogen  and  Organic  Carbon; 
Soil  Sci.,  Vol.  I,  No.  3,  pp.  197-238,  Mar.,  1916.  II.  Humus,  Humus-Ni- 
trogen and  Color;  Soil  Sci.,  Vol.  I,  No.  3,  pp.  239-258,  Mar.,  1916. 


THE  ORGANIC  MATTER  OF  THE  SOIL         121 

It  is  evident  that  humid  soils  not  only  contain  the  greater 
amounts  of  organic  matter,  but  also  excel  in  humus.  The 
humus  of  the  arid  regions,  however,  is  richer  in  nitrogen,  due 
to  the  character  of  the  decomposition  going  on.  As  a  conse- 
quence the  nitrogen  in  the  soil  of  humid  regions  is  not  greatly 
in  excess  of  that  in  the  soils  of  drier  climates.  The  percentage 
of  humus  not  only  decreases  in  the  lower  depths  of  the  soil, 
but  also  changes  in  composition,  becoming  poorer  in  nitrogen 
the  deeper  the  soil. 

64.  The  influence  of  organic  matter  on  the  soil. — The 
effects  of  organic  matter  on  soil  and  plant  conditions  are  as 
numerous  as  they  are  complex.  Some  of  the  influences  are 
direct,  others  are  indirect.  As  the  specific  gravity  of  organic 
matter  is  low,  the  first  effect  of  its  addition  would  be  to  lower 
the  specific  gravity  of  the  soil.  The  organic  matter  tends  also 
to  spread  the  individual  particles  of  soil  farther  apart,  especi- 
ally in  a  clay.  Such  action  will  markedly  influence  the  volume 
weight. 

The  loosening  effects  of  organic  matter  are  especially  ap- 
parent in  such  soil  as  clay*  On  the  other  hand,  because  or- 
ganic matter  has  a  higher  cohesive  and  adhesive  power  than 
sand,  it  performs  the  function  of  a  binding  material  with  the 
latter  soil,  a  condition  much  to  be  desired  in  a  material  pos- 
sessing such  loose  structure. 

As  the  water  capacity  of  organic  matter  is  very  high,  a  soil 
rich  in  organic  constituents  usually  possesses  a  high  water- 
holding  power.  This  makes  possible  greater  volume  changes 
both  on  drying  and  in  the  presence  of  excessive  moisture.  The 
granulating  effects  of  wetting  and  drying  and  freezing  and 
thawing  are,  therefore,  accelerated.  The  increased  water  ca- 
pacity of  the  soil,  resulting  from  the  presence  of  organic  ma- 
terials, is  of  great  importance  in  drought  resistance,  while 
the  black  color  imparted  by  the  humus  tends  to  raise  the 
heat  absorptive  power  of  the  soil.  ) 

The  better  tilth  induced  by  the  presence  of  organic  matter 


122         NATURE  AND  PROPERTIES  OF  SOILS 

in  any  soil  tends  to  facilitate  ease  in  drainage  and  to  encour- 
;iLr<'  good  aeration.  These  two  conditions  are  of  course  neces- 
sary for  the  promotion  of  soil  sanitation.  Root  extension  and 
bacterial  activity  are  thus  increased.  It  is  of  especial  impor- 
tance that  the  splitting-up  of  the  organic  matter  shall  take 
place  in  the  presence  of  plenty  of  oxygen,  in  order  that  toxic 
compounds  may  not  be  generated  and  that  products  highly 
favorable  to  plant  growth  should  be  formed. 

The  soil  organic  matter,  however,  functions  in  other  ways 
than  those  strictly  physical  and  chemical.  Its  degradation 
products  may  serve  as  nutrients  for  higher  plants.  Bacteria 
and  other  soil  organisms  are  also  furnished  a  source  of  energy 
thereby  and  the  production  of  carbon  dioxide  is  much  in- 
creased. This  carbon  dioxide,  as  well  as  the  organic  acids 
generated,  tends  to  raise  the  capacity  of  the  soil-water  as 
a  solvent,  and  thus  the  amount  of  mineral  material  available 
to  the  crop  is  greatly  increased.  The  general  effect  of  organic 
matter,  then,  is  to  better  the  soil  as  a  foothold  for  plants,  and 
to  increase  either  directly  or  indirectly  the  available  nutri- 
ent supply  for  the  crop.  i 
\f  65.  Maintenance  of  soil  organic  matter.1 — fhe  mainte- 
nance of  a  proper  supply  of  organic  matter  in  a  soil  is  a  ques- 
tion of  great  practical  importance,  as  productivity  is  gov- 
erned very  largely  by  the  organic  content  of  the  soiL|  This 
maintenance  of  the  soil  organic  matter  depends  on  two  factors : 
(1)  the  source  of  supply  and  methods  of  addition;  and  (2) 
the  promotion  of  proper  soil  conditions  in  order  that  the 

1  Snyder,  H.,  Effect  of  the  Rotation  of  Crops  upon  the  Humus  Content 
and  the  Fertility  of  Soils;  Minn.  Agr.  Exp.  Sta.  Bui.  53,  June, 
1897.  The  Production  of  Humus  in  Soils;  Minn.  Agr.  Exp.  Sta.,  Bui. 
89,  Jan.,  1905.  Morse,  F.  W.,  Humus  in  New  Hampshire  Soils;  N.  H. 
Agr.  Exp.  Sta.,  Bui.  138,  June,  1908.  Hopkins,  C.  G.,  Phosphorus  and 
Humus  in  Relation  to  Illinois  Soils;  111.  Agr.  Exp.  Sta.,  Circ.  116.  Feb., 
1908.  Thatcher,  E.  W.,  Tlie  Nitrogen  and  Humus  Problem  of  Dry 
Farming;  Wash.  Agr.  Exp.  Sta.,  Bui.  105,  June,  1912.  Fippin,  E.  O., 
Nature,  Effects  and  Maintenance  of  Humus  in  the  Soil;  Cornell  Reading 
Course  for  the  Farm,  Vol.  Ill,  No.  50,  Oct.,  1913.  Loughridge,  R.  H., 
Humus  of  California  Soils;  Calif.  Agr.  Exp.  Sta.,  Bui.  242,  Jan.,  1914. 


THE  ORGANIC  MATTER  OF  THE  SOIL         123 

organic  matter  may  perform  its  legitimate  functions.  The 
source  of  supply  will  be  considered  first. 

The  organic  matter  of  the  soil  may  be  increased  in  a  nat- 
ural way  by  the  plowing  under  of  green  crops.  This  is 
called  green-manuring  and  is  a  very  satisfactory  practice. 
Such  crops  as  rye,  buckwheat,  clover,  peas,  beans,  and  vetch 
lend  themselves  to  this  method  of  soil  improvement.  Not 
only  do  these  crops  increase  the  actual  organic  content  of 
a  soil,  but  in  the  case  of  legumes  the  nitrogen  may  also  be  in- 
creased in  amount,  if  the  nodule  bacteria  are  present  and 
active. 

Green-manures  to  be  effective  must  be  hardy,  rapid  in 
growth,  succulent,  and  should  produce  abundant  foliage.  Rye 
and  oats  are  particularly  valuable  from  this  standpoint.  Such 
legumes  as  cowpeas,  vetch,  field  peas,  soybeans,  and  velvet 
beans  are  adapted  to  summer  growth.  Red  clover  or  sweet 
clover,  being  a  biennial,  may  be  seeded  one  year  and  turned 
under  the  next  spring.  Oats  and  peas  or  rye  and  peas  make 
a  very  good  combination  for  fall  green-manuring.  Hairy  or 
winter  vetch  may  be  seeded  with  rye  in  the  autumn  and  used 
as  a  green-manure  in  the  spring.  In  the  South  green-manur- 
ing crops  may  be  utilized  to  much  better  advantage  than  in  the 
northern  states  as  the  longer  growing  season  permits  the  use 
of  a  green-manure  following  the  normal  harvest. 

Due  to  the  tendency  of  bare  soil  to  lose  nutrients  by  leach- 
ing, especially  in  the  summer  and  fall,  it  is  always  best  to 
keep  the  land  covered  with  vegetation  of  some  kind.  Cover-  or 
catch-crops  are  used  for  this  purpose,  especially  on  sandy  land, 
although  they  are  profitable  on  heavier  soils  as  well.  Wheat 
on  sandy  land  may  be  followed  by  cowpeas,  which  not  only 
conserve  nitrates  but  fix  nitrogen  from  the  air  in  addition. 
Rape,  cowpeas,  vetch,  and  soybeans  are  sometimes  seeded  in 
corn  at  the  last  cultivation.  When  a  soil  receives  clean  culti- 
vation a  part  of  the  year,  as  is  practiced  very  frequently  in 
orchards,  it  is  very  desirable  that  a  crop  be  plowed  under  oc- 


124         NATURE  AND  PROPERTIES  OF  SOILS 

casionally  to  replace  the  organic  matter  lost  by  oxidation. 
Whether  such  catch-crops  are  pastured  or  turned  under,  they 
tend  to  increase  the  soil  organic  matter.  Weeds,  which  spring 
up  after  the  crop  is  harvested,  are  often  valuable  as  cover- 
and  catch-crops  and  when  turned  under  aid  in  maintaining 
the  organic  content  of  the  land. 

Crop  residues  form  no  inconsiderable  portion  of  the  organic 
matter  produced  on  the  land.  If  such  materials  as  straw, 
stubble,  cornstalks,  and  the  like  are  incorporated  in  the  soil, 
much  will  be  accomplished  towards  the  upkeep  of  the  organic 
matter.  The  burning  of  straw  and  cornstalks,  especially  in 
the  Middle  West,  entails  an  enormous  waste  of  carbon  as  well 
as  of  nitrogen.  The  value  of  crop  residues  has  been  demon- 
strated very  conclusively  by  the  Illinois  Experiment  Station  x 
on  their  outlying  experimental  farms.  At  Bloomington,  for 
instance,  the  turning  under  of  crop  residues  for  five  years 
increased  the  wheat  yields  4.4,  7.9  and  5.9  bushels  in  1911, 
1912  and  1913  respectively. 

Farm  manure  is  one  of  the  most  important  by-products  on 
the  farm  and  is  especially  valuable  because  of  its  organic  mat- 
ter. Although  only  about  one-fourth  of  the  organic  materials 
of  the  original  food  given  the  animal  ever  reaches  the  land, 
the  use  of  such  a  by-product  is  worth  while,  since  the  carbon 
it  contains  comes  from  the  air  and  not  from  the  soil.  The  main 
losses  that  the  carbon  of  the  crop  undergoes  when  thus  util- 
ized are  due  to  the  digestive  influences  of  the  animal  and  to 
the  leaching  and  fermentation  which  goes  on  in  the  manure. 
While  sufficient  manure  ordinarily  can  not  be  produced  from 
the  crops  grown  on  the  farm  to  maintain  the  organic  matter 
of  its  soil,  the  use  of  farm  manure  with  green-manure  and  crop 
residues  in  a  proper  rotation  is  fundamental  in  good  soil  man- 
agement. 

66.     Organic  matter  and  soil  conditions. — Improper  soil 

*Mosier,  J.  G.,  and  Gustafson,  A.  F.,  Soil  Physics  and  Management, 
p.    171;    Philadelphia   and    London,    1917. 


THE  ORGANIC  MATTER  OF  THE  SOIL 


125 


conditions  not  only  prevent  the  proper  decay  of  organic  mat- 
ter, but  also  tend  to  encourage  the  production  of  products  in- 
imical to  plant  growth.  Therefore,  in  order  that  organic  ma- 
terials added  to  any  soil  may  produce  the  proper  decomposi- 
tion products  and  perform  their  normal  functions,  soil  con- 
ditions in  general  must  be  of  the  best.  Tile  drainage  should 
be  installed,  if  necessary,  in  order  to  promote  aeration  and 


SOIL 
SOIL  MINERALS- 

WATEI? 


&* 


Fig.  22. — Diagram  showing  the  practical  sources  of  the  soil  organic 
matter  and  the  cycle  through  which  its  constituents  pass.  Note 
that  the  carbon,  oxygen  and  hydrogen  come  very  largely  from  air 
and  water  and  that  fixation  of  nitrogen  may  occur  if  the  crop  is 
a  legume.  Only  about  25  per  cent,  of  the  organic  matter  fed  to 
animals  ever  reaches  the  soil  in  farm  manure  under  average  con- 
ditions. 


granulation.  Lime  should  be  added  if  basic  materials  are 
lacking,  for  it  promotes  bacterial  activity  as  well  as  plant 
growth.  The  addition  of  fertilizers  will  often  be  a  benefit, 
as  will  also  the  establishment  of  a  suitable  rotation.  The 
rotation  of  crops  not  only  prevents  the  accumulation  of  toxic 
materials,  but  also,  by  increasing  crop  growth,  makes  pos- 
sible a  larger  addition  of  organic  matter  by  green-manuring. 


126         NATURE  AND  PROPERTIES  OF  SOILS 

67.  Resume. — An  understanding  of  the  complex  organic 
relationships  within  the  soil  is  of  great  practical  value,  as 
it  determines  to  a  large  degree  the  yield  of  crops,  their  rota- 
tion order  and  their  fertilization.  Moreover,  tillage  operations 
must  be  varied  according  to  the  organic  nature  of  the  soil. 
Unless  a  system  of  soil  management  is  adopted  which  will  at 
least  partially  keep  up  the  organic  matter  of  the  soil,  crop 
yields  may  be  expected  to  decrease  materially  in  a  few  years. 

Good  soil  management  seeks  to  adjust  the  addition  of  or- 
ganic matter,  the  physical  and  chemical  condition  of  the  soil, 
and  the  losses  through  cropping  and  leaching,  in  such  a  way 
that  paying  crops  may  be  harvested  while  impairing  the  or- 
ganic supply  of  the  soil  as  little  as  possible.  Any  system  of 
agriculture  that  tends  permanently  to  lower  the  organic  mat- 
ter of  the  land  is  impractical  and  improvident,  as  well  as  un- 
scientific. 


CHAPTER  VI 
THE  COLLOIDAL  MATTER  OF  THE  SOIL1 

Research  in  physics  and  physical  chemistry  is  each  day 
making  it  clearer  that  the  properties  of  matter  are  by  no 
means  entirely  determined  by  chemical  composition.  Matter 
varies  in  its  physical  character  and  its  chemical  activities  with 
its  fineness  of  division.  Coarsely  divided  substances  function 
much  differently  when  they  become  molecular  complexes  and 
still  more  diversely  when  their  aggregates  are  divided  into 
their  molecular  and  ionic  components.  Because  of  the  par- 
ticular properties  exhibited  by  material  in  a  fine  state  of  di- 
vision, approaching  but  not  attaining  a  molecular  simplifica- 
tion, a  special  name  is  utilized.  A  substance  in  such  a  con- 
dition is  said  to  be  colloidal  or  in  the  colloidal  state. 

68.  The  colloidal  state 2  arises  when  one  form  of  matter 
(either  a  gas,  liquid,  or  solid)  in  a  very  fine  state  of  division 

1  Colloidal  chemistry  is  now  so  well  understood  that  it  will  be  necessary 
to  develop  only  those  phases  which  have  a  direct  bearing  on  soil 
phenomena. 

2 Some  of  the  following  general  references  may  prove  helpful: 

Eamann,  E.,  Kolloidstudien  bei  Bodenkundlichen  Arbeiten;  Kolloid- 
chemische  Beihefte;   Band  II,  Heft  8/9,  Seite  285-303,  1911. 

Niklas,  H.,  Die  Kolloidchemie  und  ihre  Bedeutung  fur  Boderikunde, 
Geologie,  und  Mineralogie ;  Internat.  Mitt,  fur  Bodenkunde,  Band  II, 
Heft  5,  Seite  383-403,  1913. 

Bancroft,  W.  D.,  The  Theory  of  Colloid  Chemistry;  Jour.  Phys.  Chem., 
Vol.  18,  No.  7,  pp.  549-558,  1914. 

Taylor,  W.  W.,  The  Chemistry  of  Colloids;  New  York,  1915. 

Burton,  E.  F.,  The  Physical  Properties  of  Colloidal  Solutions;  London, 
1916. 

Zsigmondy,  R.,  The  Chemistry  of  Colloids,  Part  I;  trans,  by  E.  B. 
Spear,  New  York,  1917. 

Wiegner,  G.,  Boden  und  Bodenbildung ;  Dresden  and  Leipzig,  1918. 

Bancroft,  W.  D.,  Applied  Colloidal  Chemistry;  New  York,  1921. 

Thatcher,  R.  W.,  Chemistry  of  Plant  Life;  Chap.  XV,  New  York,  1921. 

127 


128         NATURE  AND  PROPERTIES  OF  SOILS 

is  distributed  through  a  second,  which  may  also  be  a  gas,  a 
liquid,  or  a  solid.  The  material  in  the  finely  divided  state 
is  called  the  dispersed  phase,  while  the  matter  containing  it 
is  designated  as  the  continuous  or  dispersive  medium.  A 
very  good  example  of  a  colloidal  system  occurs  when  very 
fine  clay  particles  (solids)  are  suspended  in  water  (liquid) 
or  when  an  emulsion  of  oil  and  water  is  formed,  the  oil  under 
certain  conditions  becoming  the  dispersed  material,  hetero- 
geneously  disposed.  The  particles  of  material  in  a  colloidal 
state  in  these  cases  are  so  small  that  they  will  not  sink  as  long 
as  conditions  are  stable.  Moreover,  they  exhibit  the  Brownian 
movement,1  the  oscillations  increasing  very  rapidly  as  the 
size  decreases.  Such  particles  are  molecular  complexes  and 
the  solution  is  heterogeneous.  In  this  respect  a  colloidal  solu- 
tion differs  from  a  true  solution,  which  is  homogeneous,  the 
particles  being  molecules  and  often  ions. 

69.  Size  of  colloidal  particles. — The  size  of  the  particles 
of  matter  in  a  colloidal  state  vary  with  the  material  and  with 
the  conditions  of  formation.  The  diameters  of  material  in  a 
colloidal  state  are  considered  to  range  from  100  n  u  2  ( .0001 
m.m.)  to  1  \i  \i  (.000001  m.m.).  Above  100  \i  \i  suspended 
material  is  usually  sinkable,  while  below  1  [i  n  the  particles 
generally  become  single  molecules  and  a  true  solution  is  at- 
tained. Theoretically  it  would  seem  possible  to  pass  from 
a  suspension  to  a  true  solution  without  a  break  by  a  progres- 
sive subdivision  of  particles.  There  seems  to  be  a  discontinu- 
ity, however,  between  the  colloidal  state  and  a  true  solution. 
As  the  molecular  complexes  subdivide,  they  at  last  go  into 
solution  and  may  reprecipitate   as  coarser   complexes,   thus 

1  Small  particles,  even  those  well  -within  the  range  of  ordinary  micro- 
scopic vision,  exhibit,  when  suspended  in  a  liquid,  an  oscillating  motion 
around  a  central  position.  This  movement,  which  is  called  the  Brownian, 
is  inversely  proportional  to  the  size  of  the  particle.  It  is  probably  due 
to  the  bombardment  of  the  molecules  and  ions  of  the  liquid  in  which 
the  particle  is  suspended.  The  Brownian  movement  is  very  slow  for 
particles  of  a  diameter  of  .001  mm. 

aA  micron  (a)  =.001  mm.  or  10-3  mm.  A  millimicron  (l/t/t)  = 
.000001mm.  or  10-8  mm. 


THE  COLLOIDAL  MATTER  OF  THE  SOIL      129 

maintaining  a  considerable  gap  between  the  two  states  of 
matter.1 

70.  The  phases  of  a  colloidal  state. — As  already  empha- 
sized, two  phases  are  necessary  for  a  colloidal  state — a  dis- 
persive medium  and  a  material  that  will  heterogeneously 
disperse  therein.  Threee  materials  may  function  as  a  dis- 
persive medium — a  liquid,  a  solid,  or  a  gas.  In  the  same  way, 
with  each  dispersed  material  there  are  three  possibilities — 
a  liquid,  a  solid,  or  a  gas.  This  gives  eight  general  phases 
to  be  considered  in  colloidal  chemistry.2 

The  liquid-solid  and  the  liquid-liquid  phases  are  by  far 
the  most  important  as  far  as  soil  materials  are  concerned. 
The  dispersed  materials  of  soil  colloids  are  the  minerals  either 
in  a  hydrous  or  non-hydrous  condition  and  the  organic  mat- 
ter in  various  stages  of  decay.  The  dispersive  medium  is  of 
course  the  soil  solution. 

71.  Colloids  vs.  crystalloids. — It  must  not  be  inferred, 
because  the  colloidal  state  is  often  wrongly  contrasted  with 
the  crystalloidal,  that  material  in  a  colloidal  condition  is  al- 
ways amorphous.  It  is  often  crystalline.  Moreover,  it  may  be 
animate,  as  some  bacteria  are  minute  enough  to  function  col- 
loidally.  It  is  obvious  also  that  the  same  chemical  material 
may  exist  either  in  the  colloidal  or  non-colloidal  state.  For 
example,  silicic  acid,  hydrated  ferric  oxide,  gold,  carbon  black, 

1  Bancroft,  W.  T>.,  Applied  Colloidal  Chemistry,  p.  183 ;  New  York 
1921. 

2  The  eight  phases  with  examples  are : 

Solid  in  solid Carbon  in  steel. 

Liquid  in  solid water  of  crystallization 

Gas  in  solid  gases  in  minerals 

Solid  in  liquid colloidal  solution  of  metals 

Liquid  in  liquid   emulsions  of  oil  in  water 

Gas  in  liquid air  in  water,  foam 

Solid  in  gas smoke  in  air 

Liquid  in  gas clouds 

Gas  in  gas  noncolloidal,  merely  a  mixture  of 

molecules. 
After  Burton,  E.  F.,  The  Physical  Properties  of  Colloidal  Solutions, 
p.  10;  London,  1916. 


130         NATURE  AND  PROPERTIES  OF  SOILS 

and  other  materials,  may  or  may  not  be  colloidal,  according 
to  circumstances.  The  fineness  of  division  is  the  explanation 
of  colloidal  properties.  In  order  to  place  such  a  discussion  on 
a  more  understandable  basis,  a  few  additional  illustrations 
may  not  be  amiss.  The  following  materials,  which  may  exist 
in  a  colloidal  condition,  are  for  convenience  grouped  under 
two  general  heads,  organic  and  inorganic : 

Organic:  Gelatin,  agar,  caramel,  albumin,  starch  jelly, 
humus,  some  bacteria,  carbon  black,  and  tannic  acid. 

Inorganic:  Gold,  silver,  hydrated  ferric  oxide,  arsenious 
sulphide,  zinc  oxide,  silver  iodide,  Prussian  blue,  and  the  like. 

72.  The  properties  of  colloidal  materials. — In  general, 
there  are  certain  properties  which  materials  in  a  colloidal 
state  exhibit  and  by  which  they  are  distinguished  from  true 
solutions.  In  the  first  place,  since  they  are  not  in  true  solu- 
tion, they  exert  little  or  no  effect  on  the  freezing  point  and 
the  vapor  pressure  of  liquids.  Some  colloids  have  absolutely 
no  effect  on  these  properties,  while  others,  as  they  allow  a 
certain  small  amount  of  true  solution  to  take  place,  do  possess 
such  influences  to  a  slight  degree.  Secondly,  colloids  do  not 
pass  readily  through  semi-permeable  membranes,  such  as 
parchment  paper  or  pig's  bladder.  Their  diffusive  powers 
are  low.  This  serves  as  an  easy  way  of  separating  colloidal 
and  non-colloidal  material.  Thirdly,  heat  and  the  addition 
of  electrolytes  will  serve  to  coagulate  certain  colloids,  a  prop- 
erty which  again  serves  to  distinguish  them  sharply  from  a 
true  solution.  Fourthly,  colloidal  material  has  great  ab- 
sorptive power,  not  only  for  water,  but  also  for  gases  and 
materials  in  solution,  a  quality  of  extreme  importance  in  soil 
phenomena. 

Many  colloids  are  coagulated  by  the  addition  of  an  elec- 
trolyte,1 the  phenomenon  often  being  spoken  of  as  floccula- 

1  An  electrolyte  is  any  substance  which  has  the  ability  when  in  solution 
to  carry  an  electric  current,  the  substance  suffering  decomposition  there- 
by. The  current  is  carried  by  the  liberated  ions.  Hydrochloric  acid, 
for   example,   dissociates   into   ionic    hydrogen    and   ionic    chlorine,    the 


THE  COLLOIDAL  MATTER  OF  THE  SOIL      131 

tion.1  A  very  good  example  is  afforded  by  treating  a  colloidal 
clay  suspension  with  a  little  calcium  hydroxide.  The  tiny 
particles  almost  immediately  coalesce  into  floccules,  and  be- 
cause of  their  combined  weight,  sink  to  the  bottom  of  the 
containing  vessel,  leaving  the  supernatant  liquid  clear.  The 
same  action  will  take  place  in  the  soil  itself,  but  of  course  with 
less  rapidity  and  under  conditions  less  noticeable  to  the  eye. 
Some  dispersed  materials,  when  thus  separated  from  their 
dispersive  medium,  will  reassume  the  colloidal  state  with 
ease  when  an  opportunity  is  offered.  In  other  cases,  the  col- 
loidal condition  is  difficult  to  restore.  Gelatin  is  an  example 
of  the  first  group  and  is  called  a  reversible  colloid.  Ferric 
hydrate  is  an  example  of  the  more  or  less  irreversible  type. 

Just  why  this  phenomenon  of  flocculation  or  agglutination 
takes  place  is  rather  difficult  to  state.  It  is  found  that  cer- 
tain colloids,  when  subjected  to  the  proper  electric  current, 
will  migrate  to  either  the  positive  (anode)  or  the  negative 
(cathode)  pole.  These  particles  evidently  carry  a  charge  of 
electricity.  Hydrated  ferric  oxide,  aluminium  hydrate,  and 
basic  dyes,  for  example,  move  toward  the  cathode  and  carry 
a  positive  charge ;  while  arsenious  sulphide,  silicic  acid,  gold, 
silver,  humus  and  acid  dyes  move  toward  the  anode  and  are 
negative.  It  is  assumed  that  as  long  as  the  colloidal  particles 
remain  charged,  they  repel  each  other  and  the  colloidal  state 
persists.  When  an  electrolyte  is  added,  which  develops  by 
ionization  a  dominant  opposite  charge,  it  is  supposed  to  cause 
a  neutralization  of  the  repellent  electricity  carried  by  the 
colloidal  particles,  and  flocculation  occurs. 

Certain  colloids  may  flocculate  certain  others,  as  the  gela- 

tinization  of  silic  acid  by  hydrated  ferric  oxide.     At  times 

one  colloid  may  protect  another,  probably  by  surrounding  it 

former  carrying  a  positive  and  the  latter  a  negative  charge  of  electricity 
(H*-r-C_).  KN03  gives  K++ N03".  The  ionization  varies  with  the 
substance,  the  dilution  and  certain  other  conditions. 

1See  Wolkoff,  M.  I.,  Flocculation  of  Soil  Colloidal  Solutions;  Soil 
Sci.,  Vol.  I,  No.  6,  pp.  585-601,  June,  1916.  A  good  bibliography  is 
appended. 


132         NATURE  AND  PROPERTIES  OF  SOILS 

with  a  protective  film.  Such  a  case  may  be  shown  by  adding 
gelatin  to  a  clay  suspension.  When  a  colloid  such  as  hy- 
drated  ferric  oxide  is  flocculated,  it  loses  to  a  certain  extent 
its  colloidal  properties,  and  assumes  the  characteristics  of 
non-colloidal  materials. 

73.  Soil  colloids  and  their  generation.1 — In  soils  there 
seem  to  exist  two  very  general  and  indefinite  groups  of  col- 
loidal materials,  besides  all  gradations  and  variations :  ( 1 )  vis- 
cous, gelatinizing  and  reversible  colloids,  and  (2)  non-viscous, 
non-gelatinizing,  easily  coagulable  and  irreversible  colloidal 
matter.  'The  decaying  organic  materials  in  the  soil  and  the 
mineral  matter  contribute  liberally  to  both  groups.  Both 
of  these  groups,  with  their  bewildering  variations  and  grada- 
tions, play  important  parts  in  the  physical  and  chemical  phe- 
nomena of  the  normal  soil. 

The  organic  colloidal  matter  in  a  soil  rich  in  decomposing 
tissue  is  obviously  of  great  importance.  Such  material  is  very 
heterogeneous,  very  complex,  and  constantly  changing.  As 
yet  very  little  study  of  the  organic  soil  colloids  has  been  made 
because  of  the  difficulties  presented  by  the  problem.  Humus 
colloids  may  be  viscous  or  non-viscous,  as  the  case  may  be, 
and  may  or  may  not  be  thrown  down  by  calcium  hy- 
droxide. The  absorptive  power  of  these  colloids  for  water, 
gases,  and  such  materials  as  calcium,  magnesium,  and  potas- 
sium is  very  highly  developed — as  much  so,  probably,  as  that 
of  the  inorganic  colloids.  These  organic  colloids  are  not  only 
added  as  a  part  of  the  original  plant  tissue  but  are  also 
formed  during  the  tearing-down  and  splitting-off  processes 

1Van  Bemmelen,  J.  M.,  Dis  Absorption;  Seite  114-115,  Dresden,  1910. 
Also,  Die  Absorptionsverbindungen  und  das  Absorptsvermogen  der 
Ackererde;  Landw.  Ver.  Stat.,  Band.  35,  Seite  69-136,  1888 ;  Way,  J.  T., 
On  Deposits  of  Soluble  or  Gelatinous  Silica  in  the  Lower  Beds  of  the 
Chalk  Formation;  Jour.  Chem.  Soc,  Vol.  6,  pp.  102-106,  1854.  War- 
ington,  R.,  On  the  Part  Taken  by  Oxide  of  Iron  and  Alumina  in  the 
Adsorptive  Action  of  Soils;  Jour.  Chem.  Soc,  2d  ser.,  Vol.  6,  pp.  1-19, 
1868.  Cushman,  A.  S.,  The  Colloid  Theory  of  Plasticity;  Trans.  Amer. 
Cer.  Soc.,  Vol.  6,  pp.  65-78,  1904.  Ashley,  H.  E.,  The  Colloid  Matter 
of  Clay  and  its  Measurements;  U.  S.  Geol.  Survey,  Bui.  388,  1909. 


THE  COLLOIDAL  MATTER  OF  THE  SOIL      133 

incident  to  bacterial  activity,  during  which,  compounds  are 
thrown  off  in  such  a  state  of  division  as  to  assume  the  condi- 
tion that  has  been  designated  as  colloidal.  Of  course  the  chem- 
ical forces  of  weathering  are  also  operative  in  this  process  of 
organic  colloidal  production. 

While  some  inorganic  soil  colloids,  as  silicic  acid  and  hy- 
drated  ferric  oxide,  are  rather  simple  chemically,  most  of 
the  mineral  colloidal  material  is  extremely  complex.  The  soil, 
especially  when  of  a  clayey  nature,  always  contains  large 
amounts  of  complicated  hydrated  aluminum  silicates  of  con- 
stantly varying  constitution.1  Such  material,  whether  simple 
or  complex,  arises  from  ordinary  weathering  reactions  and 
develops  in  the  soil  as  the  latter  is  built  up.  A  simple  ex- 
ample may  be  cited.  When  a  feldspar  undergoes  decomposi- 
tion the  following  reaction  may  be  used  to  illustrate  the  pos- 
sible change  that  takes  place : 

2KAlSi308  +  2H20  +  C02  =  H4Al2Si209  +  4Si02  +  K2C03 

Orthoclase     Water  Carbon       Kaolinite  Silica  Potassium 

Dioxide  Carbonate 

Kaolin  almost  always  originates  in  this  way,  an  alkali  car- 
bonate and  silica  being  formed  at  the  same  time.  The  proc- 
ess is  essentially  one  of  hydration  and  carbonation;  the  car- 
bon dioxide  by  reacting  with  the  alkali  permits  the  process  to 
go  on.  The  silica  may  go  to  one  or  more  of  three  possible 
destinations,  according  to  conditions, — to  free  quartz,  to  col- 
loidal silica  or  to  make  up  complex  colloidal  hydrated  alu- 
minum silicates.    The  last  mentioned  condition  seems  the  most 

1  The  Bureau  of  Soils  have  prepared  a  colloidal  solution  from  soil 
by  passing  a  well  shaken  mixture  of  soil  and  water  through  a  Sharpies 
centrifuge.  The  colloidal  matter  was  separated  from  its  dispersive 
medium  by  means  of  a  porcelain  filter.  This  ultra-clay  seemed  to  be 
a  mixture  of  various  colloids  and  consisted  mainly  of  hydrated  alu- 
minum silicates  with  varying  amounts  of  ferric  hydroxide,  silicic  acid, 
organic  matter  and  possibly  aluminum  hydroxide. 

Moore,  C.  J.,  Fry,  W.  H.,  and  Middleton,  H.  E.,  Methods  for  Deter- 
mining the  Amounts  of  Colloidal  Material  in  Soils;  Jour.  Ind.  and 
Eng.  Chem.,  Vol.  13,  No.  6,  pp.  527-530,  June,  1921. 


134         NATURE  AND  PROPERTIES  OF  SOILS 

probable  fate  of  the  silica  as  the  process  is  strongly  one  of 
hydration. 
74.    Influence  of  colloidal  material  *  on  soil  properties. — 

*The  amount  of  matter  in  a  colloidal  state  in  soils  is  extremely 
variable,  ranging  from  almost  nothing  in  sand  to  a  very  large  percentage 
in  heavy  plastic  clays.  There  is  no  satisfactory  means  of  finding  the 
amount  of  colloidal  material  in  soil.  All  of  the  available  methods  depend 
for  their  expression  on  the  intensity  of  certain  qualities,  supposed  to 
be  developed  by  colloid  content.  This  indicates  that  the  methods  are 
largely  comparative  rather  than  exact  or  strictly  analytical  in  nature. 

Ashley 's  method  depends  on  the  absorption  of  certain  dyes  to  indicate 
the  relative  amount  of  material  in  a  colloidal  state.  The  difficulty  in  this 
method,  however,  lies  in  choosing  the  most  effective  dye  and  regulating 
its  concentration.  Moreover,  different  colloids  vary  so  much  in  absorp- 
tive capacity  for  the  same  dye,  that  only  roughly  comparative  results 
have  thus  far  been  possible. 

Mitscherlich  uses  the  absorptive  capacity  of  the  soil  for  water  vapor 
as  a  colloidal  index.  In  this  method  the  air-dry  soil  in  a  thin  layer  is 
brought  to  absolute  dryness  over  phosphorus  pentoxide.  It  is  then 
placed  in  a  desiccator  over  a  10  per  cent,  solution  of  sulfuric  acid  and 
the  condensation  is  hastened  by  a  partial  vacuum.  The  sulfuric  acid 
is  used  in  order  to  prevent  the  deposition  of  dew  on  the  soil.  After 
exposure  for  about  twenty-four  hours,  the  soils  are  found  to  have  taken 
up  their  maximum  moisture  of  condensation,  which  is  called  the  hygro- 
scopic water.  The  soil  is  then  weighed,  and  the  increase,  figured  to  a 
percentage  based  on  dry  soil,  is  taken  as  a  measure  of  colloidal  content. 
The  reverse  process  may  also  be  followed,  by  exposing  air-dry  soil  in  a 
saturated  atmosphere  and  afterwards  drying  over  phosphorus  pentoxide. 
The  hygroscopicity  of  the  soil,  or  its  hygroscopic  coefficient,  is  thus  the 
basis  for  colloidal  comparison. 

Ashley,  H.  E.,  The  Colloid  Matter  of  Clay  and  Its  Measurement; 
U.  S.  Geol.  Survey,  Bui.  388,  1909. 

Rodewald,  H.,  und  Mitscherlich,  A.  E.,  Die  Bestimmung  der  Hygro- 
skopizitat;  Landw.  Ver.  Stat.,  Band  59,  Seite  433-441,  1903.  Also, 
Mitscherlich,  E.  A.,  und  Floess,  R.,  Ein  Beitrage  zur  Bestimmung  der 
Hygroskopizitat  und  zur  Bewertung  der  physikolischen  Bodenanalyse  ; 
Internat.  Mitt.  f.  Bodenkunde,  Band  1,  Heft  5,  Seite  463-480,  1912. 

Ehrenberg,  P.,  und  Pick,  H.,  Beitrage  zur  Physikolischen  Bodenunter- 
suchung;  Zeit.  f.  Forst-  und  Jagdwesen,  Band  43,  Seite  35-47,  1911. 
Also,  Vageler,  P.,  Die  Bodewald-Mitscherlichsche  Theorie  der  Hygro- 
skopizitat vom  Standpunkte  der  Colloidchemie  und  ihr  Wert  zur  Beur- 
teitung  der  Boden;  Fuhling's  Landw.  Zeit.,  Band  61,  Heft  3,  Seite  73-83, 
1912. 

Stremme,  H.,  and  Aarnio,  B.,  Die  Bestimmung  des  Gehaltes  anorgan- 
ischer  Kolloide  in  Zersetzten  Gesteinen  und  deren  tonigen  Vnlagerungs- 
produkten;  Zeitsch.  f.  Prak.  Geol.,  Band  19,  Seite  329-349,  1911. 

Tempany,  H.  A.,  Shrinkage  in  Soils;  Jour.  Agr.  Sci.,  Vol.  VIII, 
Pt.  3,  pp.  312-330,  June,  1917. 

Beaumont,  A.  B.,  Studies  in  the  Beversibility  of  the  Colloidal  Condi- 
tion of  Soils;  Cornell  Agr.  Exp.  Sta.,  Memoir  21,  Apr.,  1919. 


THE  COLLOIDAL  MATTER  OF  THE  SOIL      135 

As  may  naturally  be  inferred  the  influence  of  the  colloidal 
matter  on  soil  conditions,  especially  as  related  to  plants,  is 
extremely  important.  This  influence  is  exerted  in  a  number 
of  ways,  modifying  the  physical  and  chemical  as  well  as  the 
biological  activities  within  the  soil. 

One  important  attribute  imparted  to  soil  by  colloid  develop- 
ment is  high  absorptive  power.  This  power  extends  not  only 
to  condensation  of  gases,  but  also  to  water  and  to  materials 
in  solution.  The  water  of  condensation  on  dry  soil  particles 
when  exposed  to  a  saturated  atmosphere  is  largely  determined 
by  the  colloidal  content.  The  absorptive  capacity  for  mate- 
rials in  solution  affects  both  bases  and  acid  radicals,  although 
the  former  is  usually  more  strongly  influenced.  This  has  a 
very  important  bearing  on  the  economic  use  of  fertilizers  and 
on  the  loss  of  plant  nutrients  from  the  soil.  Colloidal  mate- 
rial may  also  function  as  a  catalyst1  in  that  it  may  force 
certain  reactions  that  otherwise  might  proceed  but  slowly. 

Since  an  adjustment  is  always  taking  place  between  the 
soil  colloidal  material  and  the  soil  solution  as  far  as  soluble 
constituents  are  concerned,  it  is  readily  seen  that  not  only 
the  concentration  but  also  the  composition  of  the  latter  is  at 
least  partially  a  function  of  the  colloidal  matter  of  the  soil. 
Colloidal  matter,  moreover,  does  not  exert  the  same  absorptive 
power  for  all  material  but  is  capable  of  a  certain  amount  of 
selection.  For  example,  if  ammonium  sulfate  is  added  to  a 
soil,  the  ammonia  is  strongly  taken  up,  which  tends  to  release 
the  sulfate  ion.  The  continuous  use  of  such  a  fertilizer  on  a 
soil  low  in  active  bases  will  ultimately  result  in  an  acid  con- 
dition. This  is  another  example  of  the  practical  importance 
of  the  soil  colloidal  matter. 

The  movement  of  air  and  water  in  the  soil  is  strongly  in- 
fluenced by  colloidal  materials.  In  a  fine  soil  in  which  the 
individual  pore  spaces  are  normally  very  minute  the  develop- 

1  A  catalyst  is  a  material  capable  of  hastening  or  retarding  a  chemical 
reaction,  the  catalytic  agent  itself  not  entering  into  the   reaction. 


136 


NATURE  AND  PROPERTIES  OF  SOILS 


ment  of  colloidal  matter  may  seriously  interfere  with  aeration 
and  capillary  movement  of  water.  The  loosening  of  a  clay 
soil  tends  to  ameliorate  such  conditions  and  to  counteract 


0  1000         Z000  5000  4000  5000  C.C. 

Fig.  23. — Curves  showing  the  absorption  of  P04  in  parts  per  million  by 
various  soils  from  a  solution  of  mono-calcium  phosphate  containing 
200  parts  to  the  million  of  P04.  The  volume  of  the  percolate  is 
used  as  the  abscissas.  Such  absorption  is  a  rough  measure  of  the 
colloidal  content  of  a  soil. 

the  unfavorable  influence  of  the  colloidal  condition  of  the 
soil.  Such  a  structural  condition  is  largely  ascribed  to  the 
plasticity  and  cohesion1  of  the  soil,  which  are  in  turn,  of 

1  Any  material  which  allows  a  change  of  form  without  rupture  and 
which  will  retain  this  form  when  the  pressure  is  removed,  is  said  to  be 
plastic.  Putty  with  a  proper  admixture  of  oil  is  a  very  good  example 
of  a  plastic  body.  As  is  well  known,  various  materials  differ  "in 
plasticity. 

Very  closely  correlated  with  plasticity,  but  not  in  exact  similarity,  is 
cohesion.  By  the  cohesion  of  a  soil  is  meant  the  tendency  that  its 
particles  exhibit  in  sticking  together  and  in  conserving  the  mass  intact. 


THE  COLLOIDAL  MATTER  OF  THE  SOIL      137 

course,  governed  by  the  amount  and  the  quality  of  colloidal 
matter  present.1 

In  general  it  is  found  that,  other  conditions  being  equal, 
an  increase  of  certain  types  of  colloidal  matter  increases  plas- 
ticity; in  other  words,  the  ease  with  which  a  soil  may  be 
worked  into  a  puddled  condition  becomes  greater.  This  is  a 
rather  undesirable  quality  when  too  pronounced,  and  in  clays, 
in  which  it  is  most  likely  to  be  developed  because  of  the  pres- 
ence of  large  amounts  of  mineral  colloids,  some  means  of 
decreasing  the  colloidal  influence  is  advisable.  This  great 
plasticity  is  developed  because  the  colloids,  especially  those 
of  a  gelatinous  and  viscous  nature,  facilitate  the  ease  with 
which  the  particles  may  move  over  one  another  and  yet  cohere 
sufficiently  to  prevent  disruption  of  the  mass.  In  general, 
also,  the  greater  the  plasticity  of  a  soil,  the  greater  is  the 
cohesion  when  dry.  In  soils,  then,  in  which  certain  kinds  of 
colloidal  materials  are  very  high,  clodding  may  occur  if  the 
soil  is  tilled  too  dry  because  of  the  great  tendency  of  the  par- 
ticles to  cohere.  Cohesion  and  plasticity,  as  factors  in  soil 
structure,  soil  granulation,  and  tilth  will  receive  further  atten- 
tion later. 

It  must  not  be  inferred  from  the  preceding  discussion  that 
the  generation  of  colloidal  matter  is  always  detrimental  to 
soil  conditions.  In  sandy  soils  the  presence  of  such  material 
is  extremely  beneficial  as  it  tends  to  bind  the  soil  together, 
promotes  granulation,  and  prevents  loss  of  plant  nutrients  by 
leaching.  It  is  only  in  heavy  soils  in  which  excessive  amounts 
of  mineral  colloids  may  develop  that  a  detrimental  condition 
is  likely  to  exist.  This  occurs  because  of  a  high  cohesion  and 
plasticity,  because  of  the  absorption  of  plant  nutrients  and 
because  of  tendencies  toward  acidity.    The  addition  of  organic 

1  Davis,  N.  B.,  The  Plasticity  of  Clay;  Trans.  Amer.  Cer.  Soc,  Vol. 
16,  pp.  65-79,  1914.  Cushman,  A.  S.,  The  Colloid  Theory  of  Plasticity; 
Trans.  Amer.  Cer.  Soc.,  Vol.  6,  pp.  65-78,  1904.  Also,  Ashley,  H.  E„ 
The  Colloid  Matter  of  Clay  and  Its  Measurement ;  U.  S.  Geol.  Survey, 
Bui.  388,   1909. 


138         NATURE  AND  PROPERTIES  OF  SOILS 

matter  and  the  development  of  non-plastic  organic  colloids 
will  do  much  to  alleviate  such  conditions. 

75.  Resume. — The  attempt  to  explain  natural  phe- 
nomena from  the  standpoint  of  crystalloidal  chemistry  alone 
is  a  failure.  Nature  has  chosen  to  reveal  herself,  largely  in 
colloidal  form.  Such  a  condition  of  matter  is  the  rule  and 
not  the  exception.  Whether  the  sky,  the  ocean,  or  the  land 
is  dealt  with,  the  larger  part  of  the  natural  phenomena  are 
plausibly  explained  only  through  knowledge  of  colloidal  chem- 
istry. 

In  general,  the  more  complex  the  material  a.nd  the  more 
intricate  the  reactions  to  which  it  is  subjected,  the  more  likely 
it  is  that  the  colloidal  state  will  result.  Proteid  materials,  for 
example,  whether  in  plants  or  animals,  are  almost  always  col- 
loidal. It  is  to  be  expected,  therefore,  that  the  soil  with  its 
complicated  organic  and  inorganic  components  and  its  rapid 
and  complex  reactions  should  generate  colloidal  matter  and 
that  material  in  such  a  state  should  play  a  prominent  part 
in  soil  and  plant  activities. 


CHAPTER  VII 
SOIL  STRUCTURE  AND  ITS  MODIFICATION 

The  structural  condition  of  the  soil  is  very  important  to 
plant  growth,  since  the  circulation  of  air  and  water  so  nec- 
essary to  normal  development  is  controlled  thereby.  The  struc- 
tural condition  may  be  loose  or  compact,  hard  or  friable,  gran- 
ulated or  non-granulated,  as  the  case  may  be.  Of  these  con- 
ditions granulation,  especially  in  heavy  soils,  is  of  vital  im- 
portance, since  it  is  really  a  summation  of  all  favorable  struc- 
tural conditions.  By  granulation  is  meant  the  drawing  to- 
gether of  the  small  particles  around  suitable  nucleii,  so  that 
a  crumb  structure  is  produced.  The  grains  thus  cease  to 
function  singly.  The  importance  of  such  a  structural  condi- 
tion on  a  heavy  soil  is  obvious.  The  soil  becomes  loose  because 
of  the  larger  units,  air  moves  more  freely,  and  water  not  only 
drains  away  readily  when  in  excess,  but  responds  with  celerity 
to  the  osmotic  pull  of  the  plant. 

76.  Soil  structure  types. — The  structural  condition  of 
a  soil  can  generally  be  attributed  directly  to  its  textural  nature 
as  can  readily  be  seen  by  comparing  sandy  and  clayey  soils. 
For  convenience  of  discussion  two  general  structural  groups 
may  be  established:  (1)  single-grained,  and  (2)  compound- 
grained.  In  the  former  the  particles  function  more  or  less 
separately  and  the  soil  is,  as  a  consequence,  rather  open  and 
friable.  In  the  latter  group  the  particles,  being  small,  tend 
to  stick  together  and  the  units  instead  of  being  solid  are  aggre- 
gates, their  size  and  character  as  well  as  their  relations  to  each 
other  being  a  determining  factor  in  the  physical  condition  of 
the  soil.     As  most  soils  are  mixtures  of  large,  medium,  and 

139 


140         NATURE  AND  PROPERTIES  OF  SOILS 

small  particles,  it  is  only  the  coarse  sandy  soils  on  the  one 
hand  and  very  fine  clayey  soils  on  the  other  that  ideally  repre- 
sent these  two  groups.  Most  soils,  especially  loams,  present 
combinations  of  the  single  and  compound  grain  structures. 

Single-grain  structure  as  found  in  sandy  soils  has  certain 
obvious  advantages,  such  as  looseness,  friability,  good  aera- 
tion, and  drainage  and  easy  tillage.  On  the  other  hand,  such 
soils  are  often  too  loose  and  open  and  lack  the  capacity  to 
absorb  and  hold  sufficient  moisture  and  nutrient  materials. 
They  are,  as  a  consequence,  likely  to  be  droughty  and  lacking 
in  fertility.  There  is  only  one  method  of  improving  in  a  prac- 
tical field  way1  the  structure  of  such  a  soil — the  addition  of 
organic  matter.  Organic  material,  if  it  undergoes  favorable 
decomposition  when  incorporated  with  the  soil,  will  not  only 
act  as  a  binding  material  for  the  particles  but  will  also  in- 
crease the  water  capacity.  Nitrogen  also  is  added  and  if  the 
organic  matter  is  properly  supplemented  with  fertilizers  and 
lime,  the  soil  fertility  will  usually  be  markedly  improved.  A 
sandy  soil  high  in  organic  matter  is  almost  ideal  from  a  struc- 
tural standpoint. 

The  modification  of  the  structural  condition  of  a  heavy  soil 
is  not  such  a  simple  problem  as  in  the  case  of  a  sandy  one. 
In  the  latter  the  plasticity  and  cohesion  is  never  high  even 
after  the  addition  of  large  amounts  of  organic  materials  that 
rapidly  develop  into  a  colloidal  state.  In  clays  and  similar 
soils  the  potential  plasticity  and  cohesion  2  are  always  high 

1In  the  greenhouse  or  garden,  structure  may  be  modified  by  mixing 
different  soils.    This  is  not  practicable  in  the  field. 

1  There  are  no  satisfactory  methods  of  determining  either  the  plasticity 
or  the  cohesion  of  soils.  For  plasticity  determination,  see:  Atterberg, 
A.,  Dis  Plastizitat  der  Ton;  Internat.  Mitt.  f.  Bodenkunde,  Band  I, 
Heft  1,  Seite  10-43,  1911.  Kinnison,  C.  S.,  A  Study  of  the  Atterberg 
Plasticity  Method;  Trans.  Amer.  Cer.  Soc,  Vol.  16,  pp.  472-484,  1914. 

For  methods  of  estimating  cohesion: 

A  good  description  of  Schubler's  apparatus  is  found  on  page  104  of 
BodenJcunde,  by  E.  A.  Mitscherlich,  published  by  Paul  Parey,  Berlin, 
in  1905.  Haberlandt,  H.,  t>ber  die  Kohdreszenz,  Verhaltnisse  ver- 
schiedener  Bodenarten;  Forsch.  a.  d.  Gebeite  d.  Agri.-Physik.,  Band  I, 
Seite  148-157,  1878.    Also,  Wissenschaftlich  praktische  ifntersuchungen 


SOIL  STRUCTURE  AND  ITS  MODIFICATION    141 

due  to  the  presence  of  large  amounts  of  complex  hydrated 
aluminum  silicates  in  a  colloidal  condition.  The  more  plastic 
a  soil  becomes,  the  more  likely  it  is  to  puddle,1  especially  if 
worked  when  wet.  Moreover,  a  soil  of  high  plasticity  is  prone 
to  become  hard  and  cloddy  when  dry,  due  to  the  cohesive  ten- 
dencies of  the  small  particles.  Heavy  soils  must,  therefore, 
be  treated  very  carefully,  especially  in  tillage  operations.  If 
plowed  too  wet,  puddling  occurs,  the  aggregation  of  particles 
is  broken  down,  and  an  unfavorable  structure  is  sure  to  re- 
sult. If  plowed  too  dry,  great  lumps  are  turned  up  which 
are  difficult  to  work  down  into  a  good  seed-bed.  In  a  sandy 
soil,  no  such  difficulties  are  encountered.2 

Granulation  or  the  production  of  a  compound-grain  struc- 
ture is  the  only  means  of  correcting  the  physical  condition  of 
a  heavy  fine-grained  soil.  In  this  process  the  small  particles 
are  drawn  towards  innumerable  suitable  nucleii  and  a  porous 
structure  is  developed.  The  size  of  the  individual  pore  spaces 
is  thereby  increased  and  air  and  water  drainage  is  facilitated. 
The  structural  condition  in  reality  simulates  a  single-grain 
state  with  this  important  difference,  however:  the  particles 
are  porous  and  not  solid.  Unless  a  heavy  soil  possesses  at  least 
some  granulation,  it  is  more  or  less  unfit  for  agricultural 
operations.     (See  Fig.  24.) 

77.     Granulation. — "While  it  is  possible  to  list  the  factors 

auf  dem  Gebeite  des  Pflanzenbaues;  Band  I,  Seite  22,  1875.  Puchner, 
H.,  Untersuchungen  uber  die  Eohareszenz  der  Bodenarten;  Forsch.  a.  d. 
Gebiete  d.  Agri.-Physik.,  Band  12,  Seite  195-241,  1889.  Atterberg,  A., 
Die  Konsistenz  und  die  Bindiglceit  der  Boden;  Internat.  Mitt.  f.  Boden- 
kunde,  Band  II,  Heft  2-3,  Seite  149-189,  1912.  Cameron,  F.  K.,  and 
Gallagher,  F.  E.,  Moisture  Content  and  Physical  Condition  of  Soils; 
U.  S.  Dept.  Agr.,  Bur.  Soils,  Bui.  50,  1908. 

aWhen  a  soil  in  a  plastic  condition  has  been  kneaded  until  its  pore 
space  is  much  reduced  and  it  has  become  practically  impervious  to  air 
and  water,  it  is  said  to  be  puddled.  The  development  of  gelatinous 
and  viscous  colloidal  materials  seems  to  be  the  controlling  factor  in 
such  a  condition,  the  pore  space  of  a  puddled  soil  being  largely  filled 
with  such  material.  When  a  soil  in  this  condition  dries,  it  becomes  hard 
and  dense. 

2  Sandy  soils  are  often  plowed  rather  wet  in  order  to  render  them 
more  compact  than  they  normally  would  be. 


142 


NATURE  AND  PROPERTIES  OF  SOILS 


that  bring  about  granulation  in  a  soil,  it  is  difficult  to  state 
specifically  just  why  this  phenomenon  takes  place.  It  has  been 
suggested  that  much  of  the  granule  formation  in  the  soil  is 
due  to  the  contraction  of  the  moisture  around  the  particles 
when,  for  any  reason,  the  moisture  content  is  reduced.  It 
is  known  that  the  soil  particles  tend  to  be  drawn  together 
by  this  reduction  in  the  soil-moisture,  due  to  the  pulling  power 
of  the  thinned  films. 

If  to  this  condition  is  added  a  material  which  tends  to  exert 
not  only  a  drawing  power  on  loss  of  moisture,  but  also  a  bind- 


Fig.  24. — A  well  granulated  soil  and  a  puddled  soil.     Organic  matter 
plays  an  important  role  in  structural  condition. 

ing  and  cementing  power  when  dry,  all  the  essentials  for  suc- 
cessful granulation  are  present.  This  second  force  is  found 
in  the  colloidal  material  existing  in  considerable  quantities  in 
heavy  soils.  Such  materials  have  already  been  shown  to  deter- 
mine the  cohesion  of  the  soil.  The  influence  of  the  colloidal 
material  is  considered  by  many  authorities  as  the  more  im- 
portant in  the  structural  adjustments  of  the  soil. 

It  is  evident  that  if  cohesion  and  plasticity  are  to  function 
in  granulation — or,  in  other  words,  locally  in  the  soil  instead 
of  generally  and  uniformly  as  when  clodding  or  puddling 
occurs — a  certain  moisture  content  must  be  maintained.  In 
a  soil  subject  to  such  a  condition,  the  cohesive  forces  being 


SOIL  STRUCTURE  AND  ITS  MODIFICATION    143 

localized,  the  internal  strains  and  pressures  are  unequal  and 
a  tendency  arises  for  the  mass  to  divide  along  lines  of  weak- 
ness into  groups  of  particles.  The  binding  capacity  of  col- 
loidal material,  as  well  as  of  salts  deposited  from  the  soil 
solution,  tends  to  make  such  a  crumb  structure  more  or  less 
permanent.  The  moisture  content  most  favorable  for  granu- 
lation seems  to  be  that  which  is  optimum  for  plant  growth.1 

78. — Forces  facilitating  granulation.2  —  Granulation  is 
nothing  more  or  less  than  a  favorable  condition  brought 
about  by  the  force  exerted  by  a  variable  water  film  and  the 
pulling  and  binding  capacities  of  colloidal  material,  operating 
at  numberless  localized  foci.  It  is  evident  that  any  influence 
or  change  in  the  soil  which  will  cause  a  greater  localization 
of  these  operative  forces  will  promote  the  aggregation  of  the 
particles.  The  addition  of  materials  from  extraneous  sources 
is  also  a  practice  that  may  tend  to  develop  lines  of  weakness 
and  thus  cause  a  more  intense  activity  of  the  forces  at  work. 

The  conditions,  additions,  and  practices  tending  to  develop 
or  facilitate  a  granular  structure  in  soils  may  be  listed  under 
six  heads:  (1)  wetting  and  drying  of  the  soil,  (2)  freezing 
and  thrawing,  (3)  addition  of  organic  matter,  (4)  action  of 
roots  and  animals,  (5)  addition  of  lime  and  (6)  tillage.  Only 
the  last  two  need  additional  consideration. 

79.  Granulating  influence  of  lime.3 — One  of  the  effects 
of  lime  in  the  soil,  especially  of  the  oxide  and  hydroxide  forms, 

1  Cameron,  F.  K.,  and  Gallagher,  F.  E.,  Moisture  Content  and  Physical 
Condition  of  Soils;  U.  S.  Dept.  Agr.,  Bur.  Soils,  Bui.  50,  p.  8,  1908. 

2  Fippin,  E.  O.,  Some  Causes  of  Soil  Granulation;  Trans.  Amer.  Soc. 
Agron.,  Vol.  2,  pp.  106-121,  1910.  Czermak,  W.,  Ein  Beitrag  zur  Erkent- 
uis  der  Veranderungen  der  Sog  physikalischen  Bodeneig enshaften  durch 
Frost,  Hitze,  und  die  Beigabe  einiger  Salze;  Landw.  Ver.  Stat.,  Band 
76,  Heft  1-2,  Seite  73-116,  1912.  Also,  Ehrenberg,  P.,  und  Romberg, 
G.  F.  von,  Zur  FrostwirJcung  auf  den  Erdboden;  Jour.  f.  Landw. 
Band  61,  Heft  1,  Seite  73-86,  1913. 

3  Lime  in  a  strictly  chemical  sense  refers  only  to  calcium  oxide  (CaO). 
The  term  is  used  here  with  an  agricultural  meaning,  including  all  cal- 
cium and  magnesium  compounds  which  are  ordinarily  added  to  the  soil 
to  correct  acidity,  thus  including  not  only  calcium  oxide  but  calcium 
hydroxide  and  calcium  carbonate  [Ca(OH)2  and  CaC03]  as  well. 


144         NATURE  AND  PROPERTIES  OF  SOILS 

is  a  flocculating  action.  This  agglomeration,  as  already  ex- 
plained, is  the  drawing  together  of  the  finer  particles  of  a 
soil  mass  into  granules.  When  calcium  hydroxide  is  mixed 
with  water  containing  fine  particles  in  suspension  there  is 
almost  immediately  a  change  in  the  arrangement  of  the  par- 
ticles. They  first  draw  together  in  light,  fluffy  groups,  or  floc- 
cules,  which  then  rapidly  settle  so  that  the  supernatant 
liquid  is  left  clear  or  nearly  so.  This  phenomenon  is  termed 
flocculation,  because  of  the  peculiar  appearance  of  the 
aggregates.  This  flocculating  tendency  when  lime  is  added 
goes  on  in  the  soil  as  well  as  with  suspensions,  although  more 
slowly.  In  general,  the  lime  tends  to  satisfy  the  absorptive 
capacity  of  the  colloidal  material  and  by  throwing  down  these 
colloids  develops  lines  of  weakness.  The  cohesive  power  of 
the  soil  is  thus  localized  and  agglomeration  must  necessarily 
occur.  The  various  forms  of  lime  differ  in  their  flocculating 
capacities,  calcium  oxide  and  hydroxide  being  very  active, 
while  calcium  carbonate  is  relatively  inactive  in  this  regard. 

It  must  not  be  inferred  that  lime  is  generally  added  for  its 
flocculating  influence.  It  is  used  primarily  for  other  reasons, 
the  amounts  applied  being  in  general  too  small  to  have  very 
much  influence  on  the  structural  condition  of  the  soil.  War- 
ington,1  however,  reports  a  statement  of  an  English  farmer 
to  the  effect  that  by  the  use  of  large  quantities  of  lime  on 
heavy  clay  soil,  he  was  enabled  to  plow  with  two  horses  instead 
of  three.  It  is  generally  true  that  soils  rich  in  lime  are  well 
granulated,  and  maintain  a  much  better  physical  condition 
than  soils  of  the  same  texture  that  are  low  in  lime. 

80.     Tillage.2 — Tillage  aims  to  accomplish  three  primary 

1  Warington,  E.,  Physical  Properties  of  Soils,  p.  33,  Oxford,  1900. 

'For  a  very  complete  review  of  the  theory  and  practice  of  plowing 
and  cultivation,  with  a  complete  bibliography:  Sewell,  M.  C,  Tillage: 
A  Review  of  the  Literature;  Jour.  Amer.  Soc.  Agron.,  Vol.  II,  No.  7, 
pp.  269-290,  Oct.,  1919. 

The  following  books  upon  the  mechanics  of  tillage  may  prove  helpful: 

Davidson,  J.  B.,  and  Chase,  L.  W.,  Farm  Machinery  and  Farm  Motors; 
New  York,  1908. 

The  Oliver  Plow  Boole;  South  Bend,  Ind.,  1920. 


SOIL  STRUCTURE  AND  ITS  MODIFICATION    145 

purposes:  (1)  modification  of  the  structure  of  the  soil;  (2) 
disposal  of  rubbish  or  other  coarse  material  on  the  surface,  and 
the  incorporation  of  manures  and  fertilizers  into  the  soil ;  and 
(3)  the  deposition  of  seeds  and  plants  in  the  soil  in  position  for 
growth. 

The  most  prominent  of  these  purposes  is  the  modification 
of  the  soil  structure.  This  affects  the  retention  and  movement 
of  moisture  and  air,  the  absorption  and  retention  of  heat, 
and  either  promotes  or  retards  the  growth  of  organisms.  The 
creation  of  a  soil-mulch  is  merely  a  change  in  the  structure 
of  the  soil  at  such  times  and  in  such  a  manner  as  may  prevent 
the  evaporation  of  moisture.    In  fine-textured  soils,  in  which 


1  2  5 

Fig.  25. — Three  types  of  plow  bottoms;  1,  stubble;   2,  sod;  3,  general 

purpose. 

the  granular  structure  is  most  desired,  tillage  may  have  an 
important  influence  on  the  formation  or  destruction  of  gran- 
ules. As  has  been  pointed  out,  any  treatment  that  increases 
the  number  of  lines  of  weakness  in  the  soil  structure  facili- 
tates the  activities  of  the  moisture  films  and  the  colloidal  mate- 
rials in  producing  soil  granules.  Tillage  shatters  the  soil  and 
breaks  it  into  many  small  aggregates,  which  may  be  drawn 
together  and  loosely  cemented  as  a  result  of  the  evaporation 
of  moisture.  The  more  numerous  the  lines  of  weakness  pro- 
duced, the  more  pronounced  is  the  granulation;  and,  con- 
versely, the  fewer  the  lines  of  weakness  produced,  the  more 
coarse  and  cloddy  is  the  structure. 

According  to  their  mode  of  action,  tillage  implements  may 

Kamsower,  H.  0.,  Equipment  for  the  Farm  and  Farmstead;  Boston, 
1917. 
King,  F.  H.,  Physics  of  Agriculture;  Chap.  XI,  Madison,  Wis.,  1910. 


146         NATURE  AND  PROPERTIES  OF  SOILS 

be  grouped  as  follows:  plows,  cultivators,  packers  and 
crushers. 

81.  The  action  of  the  plow. — The  moldboard  plow 
brings  about  its  effects  because  of  the  differential  stresses  set 
up  in  the  furrow  slice  as  it  passes  over  the  share  and  the 
moldboard.  The  soil  in  immediate  contact  with  the  plow  sur- 
face is  retarded  by  friction,  and  the  layers  above  tend  to 
slide  over  one  another  much  as  the  leaves  of  a  book  when  they 
are  bent.  If  the  soil  is  in  just  the  proper  condition,  maximum 
granulation  results ;  but  if  the  moisture  is  too  high  or  too  low, 
puddling  or  clodding  may  follow,  especially  on  a  heavy  soil. 

Not  only  does  a  shearing  occur,  but  this  shearing  is  differ- 
ential, due  to  the  slope  of  the  share  and  especially  to  the  curve 
of  the  moldboard.  When  the  soil  is  to  be  turned  over  with 
the  least  expenditure  of  energy,  the  share  is  sloping  and  is 
set  to  deliver  a  slanting  cut,  and  the  moldboard  is  long  and 
gently  inclined.  This  allows  the  furrow  slice  to  be  turned  with 
little  granulation  and  with  a  minimum  effort.  When  maxi- 
mum granulation  and  pulverization  are  desired,  the  mold- 
board  is  short  and  sharply  turned,  and  the  share  is  less  slop- 
ing and  the  cutting  edge  less  slanting.  Such  conditions  make 
for  the  development  of  more  friction  and  the  generation  of 
those  internal  twisting  and  shearing  stresses  necessary  for 
good  granulation.  The  sharper  the  bending  of  the  furrow 
slice,  the  greater  are  the  internal  stresses  set  up.  Various 
types  of  moldboards  and  shares  designated  for  special  soils 
and  particular  operations  are  on  the  market.     (See  Fig.  25.) 

The  disc  plow  is  a  sharp  rolling  disc  set  at  such  an  angle 
that  it  slices  off  and  turns  over  the  soil,  pulverizing  it  fairly 
effectively  somewhat  after  the  manner  of  the  moldboard  plow. 
One  advantage  of  the  disc  plow  is  its  lighter  draft,  due  to 
a  rolling  rather  than  a  sliding  friction  in  the  soil.  In  prac- 
tice it  is  especially  effective  on  very  dry,  hard  soil. 

While  the  plow  is  the  very  best  pulverizing  agent  when 
optimum  soil-moisture  conditions  prevail,  it  is  also  a  most 


SOIL  STRUCTURE  AND  ITS  MODIFICATION    147 

effective  puddling  agent  when  the  soil  is  wet.  Therefore,  care 
in  the  judging  of  optimum  conditions  for  plowing  is  a  most 
important  feature  in  the  maintenance  and  encouragement 
of  soil  granulation.  A  careful  study  of  the  moisture  con- 
ditions in  a  clay  soil  is  especially  necessary  in  order  to  de- 
termine just  what  is  the  correct  moisture  content  for  good 
plowing.  That  this  condition  must  be  gauged  carefully  and 
immediate  use  made  of  the  advantages  it  offers  is  shown  by 
its  narrow  limits.  A  few  days  may  suffice  for  the  moisture  to 
pass  through  and  beyond  such  a  condition.    A  clay  soil  is  so 


Fig.  26. — A  six-shovel  cultivator. 


difficult  to  handle  at  best  that  no  opportunities  such  as  are 
offered  by  optimum  moisture  conditions  should  be  lost.  More- 
over, a  heavy  soil  plowed  too  dry  or  too  wet  does  not  regain 
its  normal  granular  condition  for  several  seasons.  Such  care 
is  unnecessary  with  a  sandy  soil. 

82.  Cultivators,  packers  and  crushers. — The  many 
types  of  cultivators  may  be  grouped  under  three  heads:  (1) 
cultivators  proper,  (2)  levelers  and  harrows,  and  (3)  seeder 
cultivators.  The  action  of  all  these  implements  is  the  same 
in  that  they  stir  the  soil,  at  the  same  time  loosening  the  struc- 
ture and  cutting  off  weeds.  While  the  action  is  much  shal- 
lower than  with  the  plow,  the  same  attention  should  be  paid 
to  moisture  conditions.    Particularly  is  this  true  in  pulveriza- 


148         NATURE  AND  PROPERTIES  OF  SOILS 

tion  immediately  after  plowing.  When  the  moisture  condi- 
tions are  optimum,  the  clods  are  more  easily  shattered  and 
the  formation  of  a  suitable  seed-bed  is  speedily  accomplished. 

The  cultivators  proper  are  well  represented  by  the  ordinary 
corn  cultivator  whether  equipped  with  shovels,  knives  or  discs. 
Under  the  leveler  and  harrow  type  may  be  placed  the  spike 
and  spring-tooth  harrow,  the  various  kinds  of  weeders,  the 
acme  harrow  and  the  disc  harrow.  The  latter  may  be  equip- 
ped with  solid,  cut-away,  or  spading  discs.  The  grain  drill, 
either  of  the  press  or  disc  type,  is  a  representative  of  the 
seeder  cultivators,  which  considerably  influence  the  structural 
condition  of  the  soil  although  such  action  is  not  their  primary 
purpose.     (See  Fig.  26.) 

Packing  and  crushing  are  ordinarily  performed  by  the  same 
implement,  since  any  tool  that  compacts  does  a  certain  amount 
of  crushing;  and,  conversely,  any  implement  that  crushes  the 
soil  does  some  compacting.  Such  an  implement  as  the  culti- 
packer  cultivates,  packs  and  promotes  granulation  in  one 
operation.  The  difficulty  of  establishing  a  rigid  classification 
is  evident. 

Rollers  may  be  of  the  solid  or  barrel  type,  the  corrugated 
type,  or  the  bar  type.  The  subsurface  packer  is  also  included 
in  this  group.  Rollers  tend  to  force  the  soil  particles  nearer 
together  and  smooth  the  surface.  If  at  the  same  time  they 
establish  a  soil-mulch  so  much  the  better.  The  rolling  of  the 
land  after  seeding  is  an  attempt  to  stimulate  the  capillary 
movement  of  the  water  and  to  hasten  germination  by  bring- 
ing the  seed  in  closer  contact  with  the  soil. 

The  planker,  drag,  or  float  is  a  common  type  of  single 
crusher.  It  is  generally  broad  and  heavy,  without  teeth  and 
is  dragged  over  the  soil.  The  lumps  are  rolled  under  its  edges 
and  ground  together  in  such  a  manner  as  effectively  to  reduce 
their  size.  The  soil  is  leveled  and  smoothed  at  the  same  time. 
This  implement  may  be  used  instead  of  a  roller  in  many  cases. 
(See  Fig.  27.) 


SOIL  STRUCTURE  AND  ITS  MODIFICATION    149 

83.  Soil  tilth. — The  previous  data  and  discussion  have 
clearly  shown  the  very  great  importance  of  structure  in  the 
successful  handling  of  the  soil  in  the  field.  Since  good  phy- 
sical condition  will  reflect  itself  on  crop  yield  it  is  evident 
that  structure  must  ultimately  be  considered  in  relation  to 
all  plant  growth.  This  relationship  is  usually  expressed  by 
the  term  tilth.  While  structure  refers  to  the  arrangement  of 
the  particles  in  general,  and  granulation  to  a  particular  aggre- 
gate condition,  tilth  goes  one  step  farther  and  includes  the 
plant.    Tilth,  then,  refers  to  the  physical  condition  of  the  soil 


Fig.  27. — A  planker  or  drag,  useful  in  the  crushing  of  clods. 

as  related  to  crop  growth.  It  may  be  poor,  medium,  good,  or 
excellent,  according  to  circumstances.  Good  tilth  may  de- 
mand in  many  soils  maximum  granulation,  in  others  only  a 
medium  development.  Tillage  operations  by  influencing  the 
structure  of  the  soil  aim  to  develop  optimum  tilth.  Optimum 
tilth  always  implies  the  presence  of  water  since  the  best  phys- 
ical relationships  cannot  be  developed  without  such  moisture 
conditions. 

84.  Summary. — The  factors  which  control  the  struc- 
tural condition  of  the  soil  to  the  greatest  extent  are  plasticity 
and  cohesion,  their  influence  intensity  being  due  directly  to 
the  presence  of  certain  kinds  of  materials,  especially  hydrated 
aluminum  silicates,  in  a  colloidal  state.  As  plasticity  and 
cohesion  increase  the  tendencies  of  a  soil  to  puddle  when  wet 


150         NATURE  AND  PROPERTIES  OF  SOILS 

and  to  clod  when  dry  are  augmented.  Therefore  in  heavy 
soils  a  modification  in  these  factors  is  advisable,  through  a 
careful  control  of  moisture  and  a  bettering  of  the  granular 
structure  of  the  soil.  Granulation,  while  due  to  some  extent 
to  the  influence  of  the  water  film,  is  traceable  largely  to  col- 
loidal matter  both  mineral  and  organic.  It  is  really  a  con- 
centration of  the  forces  of  cohesion  and  plasticity  around  num- 
berless localized  foci.  Granulation  takes  place  under  the  in- 
fluence of  wetting  and  drying,  freezing,  plants  and  animals, 
addition  of  lime  and  organic  matter,  and  tillage  operations, 
especially  plowing.  The  farmer  exerts  a  modifying  influence 
on  structure  most  efficiently  by  increasing  the  organic  content 
of  the  soil  and  by  plowing.  He  is,  of  course,  aided  and  abetted 
by  natural  forces. 

Efficient  tillage  requires  good  judgment  in  the  selection  of 
proper  implements  and  mechanical  skill  in  their  operation. 
It  demands  besides  an  understanding  of  the  properties  of  soils 
and  a  knowledge  of  their  plant  relationships.  Sandy  soils  are 
easily  handled  provided  sufficient  organic  matter  is  main- 
tained. Such  cannot  be  said  of  clayey  soils.  Due  to  the  high 
cohesion  and  plasticity  of  heavy  soils  the  moisture  zone  for 
successful  tillage  is  particularly  narrow.  The  ability  to  detect 
when  this  zone  has  been  reached  in  a  clay  soil  is  one  of  the 
essentials  of  its  successful  management.  Another  essential 
is  the  effective  widening  of  such  a  zone  by  granulation  oper- 
ations. 

The  optimum  moisture  condition  for  tillage  is  generally  near 
the  optimum  condition  for  plant  growth — a  happy  coinci- 
dence, since  by  regulating  the  moisture  content  for  plant  devel- 
opment conditions  are  rendered  most  favorable  for  all  soil  ac- 
tivities. It  is  thus  possible  to  produce  in  one  operation  that 
desideratum  in  all  soil  physical  operations,  an  optimum  tilth. 


CHAPTER  VIII 

THE  FORMS  OF  SOIL-WATER  AND  THEIR 
CHARACTERISTICS  1 

A  soil,  in  order  to  function  as  a  medium  for  plant  growth, 
must  contain  a  certain  amount  of  water.  This  moisture  pro- 
motes the  innumerable  chemical  and  biological  activities  of  the 
soil,  it  acts  as  a  solvent  and  carrier  of  nutrients,  and  in  addi- 
tion it  functions  as  a  nutrient  itself.  The  amount,  character, 
and  control  of  the  soil-moisture  must  evidently  be  reckoned 
with  in  any  study  of  soil  and  plant  relationships,  whether  they 
are  of  a  practical  or  a  theoretical  nature.  The  productivity 
of  a  soil  is  often  a  direct  function  of  its  moisture  condition. 

85.  Forms  of  soil-water. — As  has  already  been  demon- 
strated, a  soil  of  a  given  volume  weight  has  a  definite  pore 
space  which  may  be  occupied  largely  by  air  or  by  water,  or 
shared  by  both,  as  the  case  may  be.  Of  course,  an  ideal  soil 
for  growth  is  one  in  which  there  is  both  air  and  water,  the 
proportions  depending  on  the  texture  and  the  structure  of 
the  soil  and  the  character  of  the  crop.  Assuming  for  the  time 
being,  however,  that  the  pore  space  is  almost  entirely  filled 
with  water,  or,  in  other  words,  that  the  soil  is  saturated,  three 
forms  of  water  are  found  to  be  present — hygroscopic,  capillary 
and  gravitational.  These  forms  differ  not  only  in  the  amount 
and  proportion  of  the  solutes  which  they  carry  but  also  in  the 
positions  that  they  occupy  in  their  relation  to  the  larger  soil 
particles  and  the  accompanying  colloidal  complexes. 

1  Keen,  B.  A.,  Belations  Existing  Between  the  Soil  and  Its  Water 
Content;  Jour.  Agr.  Sci.,  Vol.  X,  Part  1,  pp.  44-71,  Jan.,  1920.  A 
good  review  of  the  subject. 

151 


152         NATURE  AND  PROPERTIES  OF  SOILS 

If  an  absolutely  dry  soil  is  exposed  to  a  moist  atmosphere, 
it  will  absorb  moisture  rather  rapidly  until  the  colloidal  sur- 
faces are  in  equilibrium  with  the  air  as  far  as  water  vapor  is 
concerned.  Other  conditions  being  equal,  maximum  water 
will  be  taken  up  from  an  atmosphere  which  is  saturated  with 
moisture.  The  moisture  thus  taken  up  is  called  hygroscopic 
water,  its  amount  being  determined  quite  largely  by  the  mag- 
nitude of  the  colloidal  material  present  in  the  soil. 

On  adding  more  water,  it  will  be  found  that  the  absorptive 
power  of  the  soil  has  been  by  no  means  satisfied  by  the  hygro- 
scopic water.  Moisture  will  still  be  taken  up  by  the  colloidal 
complexes  and  it  will  also  collect  in  the  interstices  between 
the  soil  particles.  This  water  which  is  above  and  beyond  the 
hygroscopic  is  generally  called  the  capillary.  That  part  held 
by  the  colloidal  complexes  is  very  similar  in  characteristics  to 
the  hygroscopic  water  in  that  it  is  tightly  held  and  is  more 
or  less  immovable.  That  portion  in  the  interstices,  especially 
the  larger  spaces,  is  in  the  form  of  a  film,  is  loosely  held,  and 
responds  to  capillary  action.  While  typical  capillary  water 
is  much  different  from  hygroscopic  moisture,  it  grades  into 
the  latter  with  no  sharp  line  of  demarcation. 

Once  the  capillary  capacity  of  the  soil  is  satisfied,  a  third 
form  of  water  may  appear.  This  water  is  but  slightly  in- 
fluenced either  by  the  colloidal  complexes  or  the  larger  soil 
particles  and  consequently  is  free  to  respond  to  the  pull  of 
gravity.  It  is  called  the  free  or  gravitational  moisture  and 
is  the  water  which  passes  through  the  soil  and  appears  in 
streams  and  rivers  bearing  in  solution  the  tremendous  amounts 
of  soluble  salts  which  are  every  year  lost  from  the  land. 

86.  Hygroscopic  water. — The  hygroscopic  water  in  a 
soil  has  been  spoken  of  as  the  water  of  condensation,  or  ab- 
sorption. It  is,  however,  quite  distinct  from  water  condensed 
on  a  surface  colder  than  the  moist  atmosphere  in  which  it  is 
placed.  All  bodies  possess  the  power,  to  a  greater  or  less  de- 
gree, of  absorbing  water  even  when  at  the  same  temperature 


THE  FORMS  OF  SOIL-WATER  153 

as  the  air  with  which  they  are  in  contact,  provided,  of  course, 
that  the  air  contains  water-vapor.  Such  condensation  is 
largely  a  function  of  the  surface  exposed. 

One  of  the  characteristics  peculiar  to  colloidal  materials  is 
a  high  absorptive  power  for  water,  whether  it  is  presented  in 
the  form  of  a  liquid  or  vapor.  This  capacity  is  due  to  the 
tremendous  surface  exposed  by  matter  in  a  colloidal  state, 
which  not  only  may  hold  the  moisture  physically  but  may 
even  force  it  into  loose  chemical  combination.1  The  hygro- 
scopic water  is  probably  not  in  the  form  of  a  film  around 
the  particles  but  in  a  much  more  intimate  relationship.  That 
which  is  held  physically  is  probably,  in  part  at  least,  in  a  con- 
dition of  solid  solution.  If  any  of  the  hygroscopic  water  is 
held  chemically,  the  bond  is  probably  a  rather  loose  one. 

A  large  proportion  of  the  hygroscopic  moisture  is  obviously 
not  in  a  liquid  state  and  consequently  is  immovable  as  such. 
When  a  hygroscopically  saturated  soil  is  exposed  to  a  partially 
saturated  air,  a  portion  of  the  hygroscopic  moisture  will  be 
lost  through  vaporization.  In  order  to  expel  the  remainder 
of  the  hygroscopic  water,  the  soil  must  be  heated.  For  con- 
venience of  determination,  it  is  generally  assumed  that  all  of 
the  hygroscopic  moisture  will  be  driven  from  an  air-dry  soil 
by  heating  it  for  four  or  five  hours  at  a  temperature  of  100° 
or  110°  C.  This  is  only  an  assumption,  however,  as  some  of 
the  moisture  in  intimate  relationship  with  the  colloidal  com- 
plexes probably  still  remains. 

The  amount  of  energy  necessary  to  expel  the  hygroscopic 
moisture  from  the  soil  is  very  great,  since  its  only  movement 
is  thermal  and  because  it  is  held  so  closely.  As  so  much 
energy  is  expended  in  removing  this  water,  it  is  reasonable  to 

1See,  Bouyoucos,  G.  J.,  Classification  and  Measurement  of  the  Dif- 
ferent Forms  of  Water  in  the  Soil  by  Means  of  the  Dilatometer  Method; 
Mich.  Agr.  Exp.  Sta.,  Tech.  Bui.  36,  Sept.,  1917.  Belationship  between 
the  TJnfree  Water  and  the  Beat  of  Wetting  of  Soils  and  its  Significance; 
Mich.  Agr.  Exp.  Sta.,  Tech.  Bui.  42,  Mar.,  1918.  A  New  Classification 
of  the  Soil  Moisture;  Soil  Sci.,  Vol.  XI,  No.  1,  pp.  33-47,  Jan.,  1921. 


154         NATURE  AND  PROPERTIES  OF  SOILS 

expect  that  a  certain  amount  of  heat  of  condensation  will  be 
apparent  when  it  is  resumed.1  Patten  2  and  Bouyoucos  3  offer 
the  following  quantitative  data  concerning  this  point : 

Table  XXVIII 

HEAT    EVOLVED    BY    WETTING    SOILS    DRIED    AT    110°  C. 


Soil 


Calories  to  a 
Kilo  of  Dry  Soil 


Quartz  sand 

Norfolk  sand 

Hagerstown  loam 

Miami  silt  loam 

Cecil  clay 

Superior  clay 

Muck  (25%  organic  matter) 
Peat  


000 
347 
1108 
1742 
3376 
5158 
6413 
22185 


87.  Determination  of  the  hygroscopic  coefficient. — 
The  methods  for  the  determination  of  the  maximum  hygro- 
scopicity  of  a  soil,  or,  in  other  words,  the  hygroscopic  coeffi- 
cient, are  simple  in  outline.  The  soil,  in  a  thin  layer,  is  ex- 
posed to  an  atmosphere  of  definite  humidity  under  conditions 
of  constant  temperature  and  pressure.  Complications  arise 
from  the  necessity  of  using  a  very  thin  layer  of  soil,  from  the 
difficulty  of  controlling  humidity,  and  from  the  tendency  of 
capillary  water  to  form  in  the  soil  interstices  before  the  hygro- 
scopic capacity  is  satisfied.  The  question  of  how  long  the 
exposure  should  take  place  has  not  been  definitely  settled.    It 

1  The  tremendous  heat  of  wetting  is  probably  due  to  the  latent  heat 
of  water,  to  the  attraction  that  soils  have  for  water  and  to  the  condition 
into  which  the  water  is  transformed.  The  heat  of  condensation  is  so 
large  as  to  suggest  the  probability  of  a  change  in  the  aggregation  of 
the  moisture  thus  absorbed. 

2  Patten,  H.  E.,  Heat  Transference  in  Soils;  U.  S.  Dept.  Agr.,  Bur. 
Soils,  Bui.  59,  p.  34,  1909. 

3  Bouyoucos,  G.  J.,  Relationship  between  the  Unfree  Water  and  the 
Heat  of  Wetting  of  Soils  and  its  Significance;  Mich.  Agr.  Exp.  Sta., 
Tech.  Bui.  42,  Mar.  1918. 


THE  FORMS  OF  SOIL- WATER  155 

is  evident,  therefore,  that  not  only  must  any  method  be  more 
or  less  arbitrary  but  that  its  value  can  only  be  comparative. 

In  the  actual  procedure,1  the  sample  of  soil  may  be  air- 
dried  or  dried  at  100°  or  110°  C.  If  the  former  method  is 
followed,  the  sample  after  exposure  is  heated  for  four  or  five 
hours  at  100°  or  110°  C,  the  loss  being  considered  as  hygro- 
scopic water.  If  oven-dried  soil  is  utilized,  the  gain  in  weight 
due  to  the  exposure  to  the  moist  air  is  the  hygroscopic  mois- 
ture. If  a  saturated  air  is  made  use  of,  the  gain  is  maximum 
hygroscopicity,  from  which  can  be  calculated  the  percentage 
of  hygroscopic  water  based  on  dry  soil,  called  the  hygroscopic 
coefficient.  If  a  partially  saturated  air  is  utilized,  a  sample 
of  stock  soil,  the  hygroscopic  coefficient  of  which  is  known,  is 
exposed  at  the  same  time.  The  determination  on  the  known 
sample  shows  what  proportion  of  possible  hygroscopic  water 
has  been  taken  up.  From  this  the  hygroscopic  coefficient  of 
the  unknown  soil  sample  can  be  calculated.2 

88.  Hygroscopic  capacity  of  soils. — Since  hygroscopic- 
ity depends  almost  directly  on  the  colloidal  nature  of  the  soil, 
it  is  evident  that  texture,  external  factors  being  under  con- 
trol, will  be  an  important  factor  in  determining  the'  hygro- 
scopic coefficient.  When  the  organic  matter  of  soils  is  more 
or  less  the  same  in  amount,  the  inorganic  colloids  seem  to  con- 

1  Hilgard,  E.  W.,  Soils;  pp.  196-201,  New  York,  1911.  This  method 
is  practically  the  same  as  that  used  for  the  comparative  estimation  of 
the  colloidal  content  of  the  soil,  the  hygroscopic  coefficient  being  the 
comparative  figure  obtained.     See  note  to  paragraph  74  of  this  text. 

Bouyoueos  determines  the  hygroscopic  coefficient  in  an  approximate 
way  by  means  of  the  dilatometer  method.  The  dilatometer  is  an 
apparatus  which  measures  the  expansion  of  water  on  freezing.  If  a  given 
amount  of  soil  and  water  is  reduced  below  zero,  the  expansion  attained 
will  reveal  the  amount  of  water  remaining  unfrozen,  due  to  its  soil 
relationships.  Bouyoueos  finds  that  the  amount  of  moisture  unfrozen 
after  supercooling  to  —4°  C.  (slightly  more  freezes  at  -78°  C.)  correlates 
fairly  well  with  the  hygroscopic  coefficient.  Bouyoueos,  G.  J.,  A  New 
Classification  of  Soil  Moisture;  Soil  Sci.,  Vol.  XI,  No.  1,  pp.  33-47, 
Jan.,  1921. 

2Alway,  F.  J.,  and  Clarke,  V.  L.,  Use  of  Two  Indirect  Methods  for 
the  Determination  of  the  Hygroscopic  Coefficients  of  Soils;  Jour.  Agr. 
Ees.,  Vol.  VII,  No.  8,  pp.  345-351,  Nov.,  1916. 


156 


NATURE  AND  PROPERTIES  OF  SOILS 


trol  the  hygroscopicity.  The  following  figures  from  Briggs 
and  Schantz,1  by  whom  the  hygroscopic  coefficient  was  deter- 
mined by  exposing  air-dry  soil  at  20°  C.  to  a  saturated  atmo- 
sphere and  then  drying  at  110°  C,  illustrate  this  point.  The 
organic  matter  was  not  a  serious  disturbing  factor. 

Table  XXIX 

HYGROSCOPIC  CAPACITY  OF  VARIOUS  SOILS  EXPRESSED  IN  PER- 
CENTAGE BASED  ON  DRY  SOIL2 


Soils 


Coarse  sand  .... 

Fine  sand 

Sandy  loam 
Fine  sandy  loam 

Loam   

Clay  loam 

Clay 


Percentage 

Hygroscopic 

of  Clay 

Coefficient 

1.6 

.5 

3.9 

1.5 

7.5 

3.5 

12.9 

6.6 

14.4 

9.6 

22.0 

11.4 

32.5 

13.2 

Griggs,  L.  J.,  and  Schantz,  H.  L.,  The  Wilting  Coefficient  for  Dif- 
ferent Plants  and  Its  Indirect  Determination;  U.  S.  Dept.  Agr.,  Bur. 
Plant  Ind.,  Bui.  230,  p.  65,  Feb.,  1912.  See  also,  Loughridge,  K,  H., 
Investigations  in  Soils  Physics;  Calif.  Agr.  Exp.  Sta.,  Kep.  of  Work  of 
the  Agr.  Exp.  Stations  of  Calif,  for  1892-3-4,  pp.  76-77.  Amnion,  Georg., 
Vntersuchungen  uber  das  Condensationsvermogen  der  Bodenconstituenten 
fur  Gase;  Forsch.  a.  d.  Gebiete  d.  Agri.-Physik.,  Band  II,  Seite  1-46,  1879. 
Dobeneck,  A.  F.,  von,  Vntersuchungen  uber  das  Absorptionsvermogen 
und  die  Uygroskopizitat  der  Bodenkonstituenten  ;  Forsch.  a.  d.  Gebiete 
d.  Agri.-Physik.,  Band  XV,  Seite  163-228,  1892. 

'During  the  many  years  of  soil  investigation,  especially  where  the 
problems  had  to  deal  either  directly  or  indirectly  with  moisture,  five 
methods  of  water  expression  have  been  evolved,  their  use  depending  on 
the  nature  of  the  work  and  on  the  points  to  be  expressed.  They  may  be 
listed  under  two  general  heads: 

A.  Percentage  expression 

1.  Percentage  on  a  dry  basis 

2.  Percentage  on  a  wet  basis 

B.  Volume  expression 

1.  Cubic  inches  to  the  cubic  foot  of  soil 

2.  Percentage  by  volume 

3.  Surface  inches 

A  soil  carrying  25  per  cent,  of  water  on  the  dry  soil  basis  contains  20 
per  cent,  on  the  moist  basis    (soil  plus  water).     The  former  method  is 


THE  FORMS  OF  SOIL-WATER 


157 


Apparently,  the  finer  the  soil,  the  higher  the  hygroscopic 
coefficient.  This  is  due  to  the  fact  that  most  of  the  inorganic 
colloidal  matter  is  carried  by  the  finer  separates.  In  consid- 
ering the  hygroscopicity,  however,  the  influence  of  the  organic 
matter  must  not  be  forgotten.  Organic  colloidal  matter  has 
a  very  marked  influence  on  absorption,  and  as  the  organic 
matter  of  the  soil  increases,  the  hygroscopicity  rises  rapidly. 
The  following  data  from  Beaumont1  is  interesting  in  this 
respect : 

Table  XXX 

THE   HYGROSCOPIC    COEFFICIENT2    COMPARED    TO   CERTAIN    OTHER 

SOIL  FACTORS 


Soil 

Clay 

% 

Igni- 
tion 

% 

Humus 
% 

Hygro- 
scopic 
Coeffi- 
cient 

% 

Dunkirk  silty  clay  loam,  surface 
Dunkirk  silty  clay  loam,  subsoil 

Clyde  clay  loam,  surface 

Vergennes  clay,  subsoil 

12.9 
20.0 

20.1 
74.5 

5.08 
3.05 

14.54 
5.79 

1.26 
.20 

4.34 
.49 

3.80 
5.77 

18.90 
17.40 

In  comparing  the  two  Dunkirk  soils  it  is  apparent  that  the 
colloidal  clay  is  the  dominant  factor  in  determining  the  mag- 
preferable  in  that  the  basis  for  calculation  is  not  a  changeable  one  as  is 
the  weight  of  moist  soil.  The  dry  basis  is  practically  always  used  in 
soil  work. 

Where  two  soils  of  different  volume  weight  are  compared,  the  per- 
centage relationship  does  not  give  a  true  idea  of  the  relative  amounts 
of  water  present.  A  volume  expression  should  then  be  used.  If  a  cubic 
foot  of  soil,  weighing  100  pounds,  contains  10  pounds  of  water  it  would 
be  carrying  (10  X  27.6)  or  276  cubic  inches  of  water.  This  would 
equal  (276 -f- 1728)  X  100  or  I5-9  per  cent,  by  volume  or  (10-^5.2)  = 
1.92  surface  inches. 

1  Beaumont,  A.  B.,  Studies  in  the  Reversibility  of  the  Colloidal  Condi- 
tion of  Soils;  Cornell  Agr.  Exp.  Sta.,  Memoir  21,  pp.  501-504,  April, 
1919. 

'Moisture  content  in  this  text  unless  otherwise  indicated  will  always 
be  expressed  on  the  dry  soil  basis. 


158         NATURE  AND  PROPERTIES  OF  SOILS 

nitude  of  the  hygroscopic  coefficient.  With  the  Clyde  and 
Vergennes,  however,  the  organic  colloidal  matter  is  dominant, 
since  the  Clyde  with  only  20  per  cent,  of  clay  has  a  higher 
hygroscopic  figure  than  the  Vergennes  which  carries  74.5  per 
cent,  of  that  separate.  The  Clyde  clay  loam  and  the  Dunkirk 
subsoil  have  the  same  amount  of  clay,  yet  the  former  pos- 
sesses a  hygroscopic  coefficient  over  three  times  larger. 

Two  external  conditions  seem  to  be  important  in  determin- 
ing the  amount  of  hygroscopic  water  in  soils — (1)  humidity 
and  (2)  temperature.  It  has  been  definitely  established  that 
the  higher  the  humidity  the  higher  the  content  of  hygro- 
scopic moisture.  An  air-dry  soil  will,  therefore,  contain  less 
moisture  in  a  dry  atmosphere  than  in  one  carrying  large 
amounts  of  water-vapor.  When  the  soil  is  in  contact  with  a 
saturated  air  it  will  take  up  hygroscopic  water  to  its  full 
capacity  and  be  at  the  point  spoken  of  as  the  hygroscopic 
coefficient.  As  the  soil  air  is  generally  considered  to  be  satu- 
rated or  almost  saturated  with  water-vapor,1  except  in  the 
surface  layers  or  during  periods  of  protracted  drought,  a  soil 
in  normal  condition  may  be  considered,  for  all  practical  pur- 
poses, to  be  at  its  maximum  hygroscopicity.  An  increase  of 
the  temperature  of  the  saturated  atmosphere  seems  to  increase 
hygroscopicity.  With  a  partially  saturated  air  the  influence 
seems  to  be  in  the  opposite  direction.2  This,  however,  is  not 
an  important  practical  point. 

The  hygroscopic  coefficient,  defined  as  the  maximum  hygro- 
scopic water  that  a  soil  will  hold,  is  controlled  largely  by  the 
texture  and  organic  content  of  the  soil.  It  may  vary  from  a 
very  low  figure  in  a  sandy  soil  to  as  high  as  15  per  cent,  for 
a  clay  high  in  organic  matter.    With  a  muck  or  peat,  the  per- 

1  Russell,  E.  J.;  and  Applyard,  A.,  The  Atmosphere  of  the  Soil:  Its 
Composition  and  Causes  of  Variation;  Jour.  Agr.  Sci.,  Vol.  VII,  Part  1, 
p.  5,  1915. 

2  For  a  full  discussion  of  this  point,  see  Lipman,  C.  B.,  and  Sharp, 
L.  T.,  A  Contribution  to  the  Subject  of  the  Hygroscopic  Moisture  of 
Soils;  Jour.  Phys.  Chem.,  Vol.  15,  No.  8,  pp.  709-722,  Nov.,  1911. 


THE  FORMS  OF  SOIL-WATER  159 

centage  would  be  considerably  higher,  in  some  cases  reaching 
50  or  60  per  cent.  It  must  always  be  kept  in  mind,  however, 
that  the  point  designated  as  the  hygroscopic  coefficient  is  more 
or  less  arbitrary  and  that  there  is  no  sharp  line  of  demarca- 
tion between  the  moisture  designated  as  hygroscopic  and  that 
which  lies  near  it,  but  is  called  capillary. 

89.  The  capillary  water.1 — The  moisture  above  the 
hygroscopic  coefficient  but  not  free  to  respond  to  gravity  is 
generally  spoken  of  as  the  capillary  water.  The  portion  of 
this  moisture  lying  in  contact  or  in  the  immediate  neighbor- 
hood of  the  hygroscopic  water  is  probably  capable  of  only 
sluggish  diffusion  movement  if  any.2  This  part  of  the  capillary 
moisture  is  held  largely  by  the  colloidal  matter  and  may  be 
considered  as  transitional  between  the  true  hygroscopic  and 
the  more  active  capillary  portion.  Although  so  closely  related 
to  the  hygroscopic  water  in  general  properties  and  character- 
istics, the  soil  does  not  assume  it  by  absorption  from  vapor- 
laden  air.  This  separates  it  at  least  analytically  from  the 
hygroscopic  form  of  moisture.  Moreover,  it  is  probably 
largely  in  the  liquid  state,  which  is  hardly  true  of  all  of  the 
hygroscopic  water. 

The  more  active  capillary  water  exists  in  the  large  inter- 
stices and  as  a  film  over  the  particles  and  the  colloidal  com- 
plexes. It  is  held  rather  loosely  by  the  soil,  yet  strongly 
enough  to  counteract  gravitation.  This  part  of  the  capillary 
moisture,  being  more  or  less  beyond  colloidal  influence,  is 
free  to  respond  to  the  forces  active  in  true  solutions  and,  there- 
fore, may  move  from  place  to  place  as  equilibrium  stresses 
may  demand.  "While  the  inner  portion  of  the  capillary  water 
is  held  by  the  absorptive  power  of  the  colloidal  surfaces,  the 
outer  and  freer  portion  is  maintained  by  the  surface  tension 

1  The  colloidal  conceptions  regarding  soil-moisture  has  made  it  advis- 
able to  give  the  term  capillary  a  broader  significance  than  its  root 
meaning  justifies. 

a  Bouyoucos,  G.  J.,  A  New  Classification  of  the  Soil  Moisture;  Soil 
Sci.,  Vol.  XI,  No.  1,  pp.  33-47,  Jan.,  1921. 


160        NATUBE  AND  PROPERTIES  OF  SOILS 

of  the  water  film.  The  distinctive  characteristics  of  these  two 
portions  of  the  capillary  water  are  due  to  their  controls — 
colloidal  in  one  case,  surface  tensional  in  the  other.1 

While  the  outer  portion  of  the  capillary  water  is  undoubt- 
edly in  the  form  of  a  more  or  less  continuous  film  from  par- 
ticle to  particle,  the  bulk  of  such  moisture  probably  exists 
normally  in  the  interstices  between  the  soil  grains.  Such  a 
condition  arises  because  of  the  pressure  developed  by  the 
force  of  surface  tension.  The  pressure  due  to  surface  tension, 
however  it  may  be  expressed,  varies  with  the  curvature  of 
the  film  and  is  proportional  to  twice  the  surface  tension  di- 
vided by  the  radius.  The  less  the  radius  the  greater  the  cur- 
vature and,  therefore,  the  greater  the  stress  developed  by  sur- 
face tension.2 

The  situation  so  far  as  the  soil  is  concerned  may  be  ex- 
plained in  an  empirical  way  as  follows :  Suppose  that  two  par- 
ticles, each  carrying  a  capillary  water  film,  be  brought  into 
such  contact  that  the  films  coalesce.  There  are  now  two 
distinct  surfaces,  that  at  A,  A'  (see  Fig.  28),  with  the  curva- 

1Bouyoucos  classifies  these  two  types  of  capillary  water  as  free  (the 
more  active)  and  capillary-absorbed  (the  inner  group).  The  distinction 
is  made  on  the  basis  of  his  dilatometer  results,  the  portion  which  freezes 
at  about  0°C  being  considered  as  the  more  active  or  free. 

Bouyoucos,  G.  J.,  A  New  Classification  of  the  Soil  Moisture;  Soil 
Sci.,  Vol.  XI,  No.  1,  pp.  33-47,  Jan.,  1921. 

"Surface  tension  is  the  tension  of  a  liquid  surface  by  virtue  of  which 
it  acts  like  an  elastic  enveloping  membrane,  tending  always  to  contract 
to  the  minimum  area.  While  molecules  in  the  interior  portion  of  the 
liquid  are  attracted  in  all  directions  and  are  thus  at  equilibrium,  those 
on  the  surface  are  attracted  by  an  overbalancing  force  toward  the 
interior.  In  measurement,  surface  tension  is  considered  as  the  force  with 
which  the  surface  on  one  side  of  a  line,  one  centimeter  long,  pulls  against 
that  on  the  other  side  of  the  line.  It  is  generally  expressed  in  dynes. 
The  pressure  due  to  surface  tension  varies  with  the  curvature  of  the  film. 
It  is  usually  expressed  as: 

p  =  2T 
r 
where  P  is  the  pressure;   T,  surface  tension;   and  r,  the  radius  of  the 
drop.    As  the  radius  becomes  less,  the  curvature  increases  and  the  pres- 
sure due  to  surface  tension  increases.     An  increase  of  T  will  increase 
the  pressure,  P.  v 


THE  FORMS  OF  SOIL-WATER 


161 


ture  of  the  original  film,  and  that  at  B,  which  is  very  acute 
and  which  naturally  must  exert  a  very  great  outward  pull. 
Under  the  stress  of  this  pull  developed  by  the  surface  tension 
acting  in  this  film  of  very  great  curvature,  the  water  is  drawn 
into  the  space  between  the  particles,  where  it  becomes  thicker 
than  the  capillary  film  about  the  particles.  The  readjustment 
continues  until  the  forces  developed  by  the  two  films  become 
equal.  An  equilibrium  is  now  established.  In  the  soil  the 
tendency  towards  adjustment  is  somewhat  similar  in  so  far 


Fig.  28. — A  conventional  diagram  showing  the  coalescence  and  read- 
justment of  the  outer  capillary  water  film  of  two  particles  when 
Drought  in  contact.  At  the  left  is  shown  the  condition  before  the 
adjustment  with  a  sharp  angle  at  B;  on  the  right,  the  films  are  at 
equilibrium  with  a  thickening  at  B  due  to  movement  from  A  and  A'. 


as  the  outer  capillary  water  is  concerned.  Complete  equilib- 
rium is  probably  never  reached,  however,  due  to  constantly 
disturbing  factors. 

90.  The  determination  of  the  amount  of  capillary 
water  in  the  soil. — The  capillary  water  in  a  sample  of 
field  soil  may  be  determined  by  making  a  moisture  test  in  the 
ordinary  way  for  the  total  water  contained,1  after  the  gravi- 

1  A  moisture  determination  on  a  sample  of  field  soil  is  generally  carried 
out  as  follows: — 100  grams  of  the  sample,  after  thorough  mixing,  is 
weighed  into  a  suitable  weighing  dish  and  air-dried.  The  sample  is  then 
placed  in  an  oven  and  heated  at  100°C  or  110°C  for  four  or  five  hours. 
It  is  then  cooled  in  a  disiccator  and  weighed.  The  loss  in  weight  is 
water.  The  moisture  is  calculated  as  percentage  based  on  the  dry  mat- 
ter of  the  soil.  If  the  weight  of  the  water  lost  was  20  grams,  the 
percentage  of  moisture  would  be  (20  -j-  80)  X  100  or  25  per  cent  based 
on  dry  soil. 


162         NATURE  AND  PROPERTIES  OF  SOILS 

tational  water  has  had  time  to  drain  away.  This  represents 
the  hydroscopic  plus  the  capillary  water.  A  determination 
of  the  hygroscopic  coefficient  on  another  sample  yields  a  figure 
which,  when  subtracted  from  the  total  water,  will  give  the 
capillary  water  present  in  the  soil.  The  capillary  water  at 
various  points  in  a  soil  column  may  be  obtained  by  subtracting 
the  hygroscopic  coefficient  from  the  various  percentages  of 
moisture  present,  since  the  hygroscopic  moisture  is  little  in- 
fluenced by  height  of  column  or  ordinary  structural  condi- 
tions. 

The  determination  cited  above  may  or  may  not  give  the 
maximum  water-holding  capacity  of  a  soil.  To  fill  such  a  need 
a  laboratory  method  has  been  devised  by  Hilgard,1  which 
attempts  to  show  the  maximum  retentive  power  of  a  soil  for 
water. 

A  small  perforated  brass  cup  is  used,  having  a  diameter 
of  about  5  centimeters  and  capable  of  containing  a  soil  column 
1  centimeter  in  height.  A  short  column  is  used,  since  it  is 
only  under  such  conditions  that  a  soil  may  retain  against 
gravity  the  greatest  amount  of  water.  Also  the  soil  is  able 
to  expand  or  contract,  as  the  case  may  be,  on  the  assumption 
of  water  until  an  equilibrium  is  reached.  A  filter-paper  disc 
is  often  placed  in  the  metal  cup,  and  the  soil  is  poured  in, 
gently  jarred  down,  and  stroked  off  level  with  the  top  of  the 
cup.  The  cup  is  then  set  in  water  and  the  soil  is  allowed  to 
take  up  its  maximum  moisture.  After  draining,  the  weight 
of  the  wet  soil  plus  the  cup,  together  with  the  weights  pre- 
viously obtained,  will  allow  a  calculation  of  the  total  water 
retained  based  on  the  absolutely  dry  soil.  If  the  maximum 
capillary  water  is  desired,  the  hygroscopic  coefficient  may  be 
subtracted  from  the  maximum  water  retained. 

Since  this  method  is  a  laboratory  procedure  and  the  soil 
used  is  not  in  its  normal  structural  state,  the  results  cannot 
be  accurately  applied  to  field  conditions.     While  the  figures 

1  Hilgard,  E.  H.,  Soils,  p.  209,  New  York,  1911. 


THE  FORMS  OF  SOIL-WATER  163 

obtained  may  be  fairly  accurate  for  a  sand,  they  are  certainly 
much  too  high  for  heavy  soils.  Comparisons  with  field  soils 
have  shown  the  data  obtained  by  the  above  method  to  be  from 
30  to  130  per  cent,  too  high.1 

91.  The  capillary  capacity  of  soils. — As  might  nat- 
urally be  expected,  the  factors  that  tend  to  vary  the  amount 
of  capillary  water  in  a  soil  are  several  and  their  study  is 
rather  complex  due  to  the  secondary  influences  that  they  may 
generate  and  to  the  variable  nature  of  the  capillary  moisture. 
These  factors  may  be  discussed  under  four  heads :  ( 1 )  surface 
tension,   (2)  texture,   (3)  structure  and  (4)  organic  matter. 

Any  condition  that  will  influence  surface  tension  will  ob- 
viously influence  the  forces  active  in  the  outer  portion  of  the 
capillary  water.  A  rise  in  temperature,  for  example,  if  the 
soil  is  capillarily  saturated,  will  allow  some  of  the  water  to 
become  gravitational.  A  lowering  of  temperature  would  cause 
an  opposite  change.  This  theory  has  been  verified  by  certain 
experiments  by  King,2  in  which  he  found,  other  conditions 
being  constant,  a  very  decided  influence  on  capillary  water 
through  change  of  temperature.  Wollny  3  has  shown  that  a 
depression  of  .  65  per  cent,  in  sand  to  as  high  as  3 . 7  per  cent, 
in  kaolin  may  occur  from  a  rise  in  temperature  of  twenty 
degrees.  While  surface  tension  may  be  greatly  varied  by  the 
presence  of  salts  in  solution,  the  soil-water  is  generally  so 
dilute  that  the  condition  is  not  very  important  4  in  determining 

1  Alway,  F.  J.,  and  McDole,  G.  R.,  The  'Relation  of  Movement  of 
Water  in  a  Soil  to  its  Eygroscopicity  and  Initial  Moistness;  Jour.  Agr. 
Res.,  Vol.  X,  No.  8,  pp.  391-428,  1917. 

Israelson,  O.  W.,  Studies  on  Capacities  of  Soils  for  Irrigation 
Water;  Jour.  Agr.  Res.,  Vol.  XIII,  No.  1,  pp.  1-36,  1918. 

2  King,  F.  Hv  Fluctuations  in  the  Level  and  Bate  of  Movement  of 
Ground  Water;  U.  S.  Dept.  Agr.,  Weather  Bur.,  Bui.  5,  pp.  59-61, 
1892. 

3  Wollny,  E.,  Untersuckungen  iiber  die  WasserJcapacitat  der  Bodenarten; 
Forsch.  a.  d.  Gebiete  der  Agri.-Physik,  Band  9,  Seite  361-378,  1886. 

4Karraker,  P.  E.,  Effect  on  Soil  Moisture  of  Ckanges  in  tke  Surface 
Tension  of  tke  Soil  Solution  Brought  About  By  Addition  of  Soluble] 
Salts;  Jour.  Agr.  Res.,  Vol.  4,  No.  2,  pp.  187-192,  May,  1915. 


164         NATUKE  AND  PROPERTIES  OF  SOILS 

capillary  capacity  except  in  arid  or  semi-arid  regions.  In 
fact,  changes  in  surface  tension  through  any  cause  are  of  little 
practical  importance. 

The  finer  the  texture  of  a  soil  the  higher  is  its  capillary 
capacity.  This  is  due  to  the  presence  of  colloidal  material 
and  to  the  greater  number  of  angles  in  which  capillary  water 
may  be  held.  The  amount  of  internal  surface  exposed  by  a 
fine-textured  soil  is  immensely  larger  than  in  one  of  a  sandy 
character.  While  texture  influences  both  the  inner  and  outer 
capillary  water  the  structure  of  the  soil  has  more  to  do  with 
the  active  film-like  portion.  As  a  clayey  soil  is  granulated 
the  interstitial  spaces  are  enlarged  and  an  increased  capillary 
capacity  results.  At  the  same  time,  compacting  a  sand  will 
cause  a  rise  in  the  capillary  capacity  of  that  soil  by  increasing 
not  only  the  actual  effective  surface,  but  also  the  number  of 
angles  possible  for  capillary  concentration.  Further  compact- 
ing will  then  cause  a  decrease. 

Organic  matter,  especially  when  well  decayed,  is  commonly 
recognized  as  having  great  capillary  capacity,  far  excelling 
the  mineral  portion  of  the  soil  in  this  respect.  Its  porosity 
affords  an  enormous  internal  surface,  while  its  colloids  exert 
an  affinity  for  moisture  which  raises  its  water  capacity  to  a 
very  high  degree.  Its  tendency  to  swell  on  wetting  is  but  a 
change  in  condition  incident  to  an  approach  to  its  maximum 
moisture  content,  and  has  a  very  marked  influence  on  the 
structure  of  the  soil.  The  water-holding  capacity  of  muck 
and  peat  may  range  as  high  as  300  or  400  per  cent,  based  on 
the  dry  matter  present.  Assuming  a  hygroscopic  coefficient 
of  50  per  cent.,  the  capillary  figure  is  still  very  high.  Besides 
this  direct  effect,  organic  matter  exerts  a  stimulus  toward 
better  granulation,  a  condition  in  itself  favorable  to  increased 
water-holding  power. 

The  capillary  water  in  any  soil,  other  conditions  being  equal, 
tends  to  vary  with  the  height  of  the  column.  This  comes  about 
from  the  effect  of  gravity  on  the  outer  portion  of  the  capillary 


THE  FORMS  OF  SOIL-WATER 


165 


film,  tending  to  give  more  water  at  the 
base  of  the  column. 

The  condition  may  be  explained  em- 
pirically as  follows:  If  a  number  of  par- 
ticles carrying  maximum  capillary  films 
are  brought  together  vertically,  the  weight 
of  a  large  portion  of  the  conducting  film 
is  thrown  momentarily  on  the  surfaces  at 
the  top.  The  capillary  spaces  at  this  point 
immediately  lose  water  downward,  so  that 
they  may  assume  a  greater  curvature  and 
thus  support  this  extra  weight  thrown  on 
them.  This  curvature  must  be  sufficient  to 
balance  the  curvature  pressure  of  the  par- 
ticles below  plus  the  weight  of  the  water 
in  the  connecting  films.  The  particles  be- 
neath are  at  the  same  time  undergoing  a 
similar  adjustment  with  a  set  of  particles 
farther  below,  losing  water  in  order  to 
allow  a  change  of  curvature.  The  action 
continues  in  this  manner  in  an  attempt  to 
establish  equilibrium,  thus  giving  more 
water  at  the  bottom  of  the  column.  If  the 
amount  of  capillary  water  is  too  great  to  be 
supported,  enough  is  lost  by  gravity  to 
bring  about  an  equilibrium  (see  Fig.  29). 

The  above  illustration,  however,  does  not 
apply  strictly  to  soil  conditions,  since  only 
part  of  the  capillary  water  is  in  a  true  film 
form  and  free  to  move  with  extreme  ease. 
Moreover,  rain  water  is  applied  from 
above,  where  also  occurs  rapid  evaporation. 
Thus  at  any  particular  time  the  moisture 
content  of  a  field  soil  might  be  higher  near 
the  surface  than  farther  down  in  the  soil 


Fig.  29. — Diagram 
showing  in  a  con- 
ventional way 
the  adjustment 
tendency  of  the 
outer  capillary 
water  in  a  long 
column  and  the 
appearance  of 
free  water  if  the 
weight  is  too 
great. 


166 


NATURE  AND  PROPERTIES  OF  SOILS 


or  vice  versa  as  the  case  may  be.  As  the  capillary  water  in  a 
soil  is  reduced  there  is  a  tendency  for  the  soil  column  to  be 
more  nearly  uniform,  providing,  of  course,  that  the  equi- 
librium forces  have  had  time  to  act  and  are  not  too  much 
influenced  by  other  factors. 

While  representative  data  regarding  the  moisture-holding 
capacity  of  soils  are  difficult  to  give,  the  following  figures 
from  Alway1  indicate  the  general  effect  of  texture  and  organic 
matter.  The  maximum  water  capacity  was  determined  in  the 
laboratory  and  the  maximum  field  capacity  was  obtained  by 
sampling  the  soils  very  shortly  after  irrigation. 

Table  XXXI 

THE  MAXIMUM  WATER  CAPACITY  OF  VARIOUS   SURFACE  SOILS  AS 

DETERMINED  IN  THE  LABORATORY  AND  UNDER  FIELD 

CONDITIONS,  RESPECTIVELY  2 


Soils 

Organic 

Matter 

% 

Hygro- 
scopic Co- 
efficient 

% 

Field 

Water 

Capacity 

% 

Maximum 

Water 

Capacity, 

Laboratory 

Method 

% 

Sand 

1.22 
1.07 
1.55 
4.93 
2.22 

1.1 
1.7 
3.3 
10.0 
10.1 
10.2 
12.9 

11.7 
12.8 
19.6 
31.5 
31.3 
39.2 
47.6 

37.0 

Sand 

27.1 

Sandy  soil,  residual. 
Red  loam,  residual .  . 

Silt  loam,  loess 

Silt  loam,  loess 

Black  adobe 

34.2 
49.0 
56.8 
60.9 
60.3 

The  effect  of  texture  on  water  capacity  is  very  apparent,  a 
rough  correlation  existing  also  between  the  water  retained  and 
the  hygroscopic  coefficient.     The  influence  of  organic  matter 

1  Alway,  F.  J.,  and  McDole,  G.  K.,  The  Relation  of  Movement  of 
Water  in  a  Soil  to  its  Hygroscopicity  and  Initial  Moistness);  Jour. 
Agr.  Res.,  Vol.  X,  No.  8,  pp.  391-428,  1917. 

aNote  again  that  moisture  percentages  are  always  expressed  on  dry- 
soil  weight.     ' 


THE  FORMS  OF  SOIL-WATER 


167 


is  clearly  shown  by  the  two  loess  silt  loams.  Perhaps  most 
important  of  all  is  the  marked  discrepancy  between  the  actual 
field  capacity  and  the  arbitrary  and  artificial  laboratory 
method.  The  normal  water-holding  capacity  of  a  mineral  soil, 
varying  with  texture  and  organic  matter,  seems  to  range  from 


50  %WATER 


Fig.  30. — Diagram  showing  the  distribution  of  moisture  in  capillary 
columns  of  soil  of  different  textures.  The  end  of  each  column 
rests  in  free  water.     (Buckingham,  E.,  Bur.  Soils,  Bui.  38,  1907.) 

about  10  to  50  per  cent,  based  on  dry  soil.  Muck  and  peat  of 
course  run  much  higher,  400  per  cent,  being  not  uncommon.1 

JBriggs  and  McLane  have  perfected  a  method  of  comparing  soils  on 
the  basis  of  their  capacity  to  hold  water  against  a  definite  and  constant 
centrifugal  force  of  one  to  three  thousand  times  the  force  of  gravity. 
The  soils,  in  thin  layer,  are  placed  in  perforated  brass  cups  which  fit 
into  a  centrifugal  machine  capable  of  developing  the  above  force,  and 
are  whirled  until  equilibrium  is  reached.  The  resultant  moisture  per- 
centage is  designated  as  the  moisture  equivalent.  It  really  represents 
the  capillary  capacity  of  a  soil  of  minimum  column  length  when  subject 
to  a  constant  and  known  force  or  pull.  The  finer  the  soil,  the  greater 
of  course  is  the  moisture  equivalent.  The  authors  found  that  1  per  cent. 
of  clay  or  organic  matter  represented  a  retentive  power  of  about  .62 


168         NATURE  AND  PROPERTIES  OF  SOILS 

92.  Capillary  movement  of  water. — It  has  already  been 
shown  that  different  thicknesses  of  capillary  films  tend  to 
equalize  in  the  soil  due  to  the  pulling  forces  developed  by  the 
angle  of  curvature  between  the  particles.1  It  is  evident  that 
differences  in  curvatures  must  be  the  motive  force  in  the  capil- 
lary movement  of  soil-water.  Let  it  be  supposed,  for  conveni- 
ence, that  three  equal  spheres  when  brought  in  contact  contain 
unequal  amounts  of  water  in  the  angles  of  curvature  (see 
Fig.  31).  In  this  case  the  greater  pull  would  exist  at  A,  since 
the  angle  here  is  more  acute.    Consequently  water  must  move 


per  cent.,  while  1  per  cent,  of  silt  corresponded  to  a  retention  of  only 
.13  per  cent,  of  water.    Eepresentative  data  is  as  follows: 


Soils 

Organic 

Matter 

% 

Sands 

% 

Silt 

% 

Clay 

% 

Moisture 
Equivalent 

% 

Norfolk  coarse  sand... 
Norfolk  fine  sandy  loam. 
Yazoo  loam 

.9 
1.3 
1.3 
2.0 
3.7 
1.4 

87.9 
73.4 
25.8 
14.9 
30.9 
10.0 

7.3 

18.1 
64.1 
62.9 
42.5 
56.6 

4.8 
8.5 
10.1 
22.2 
26.6 
33.4 

4.6 

6.8 

18.9 

Waverly  silt  loam 

Houston  clay  loam.  . . . 
Houston  clay 

24.4 
32.4 
38.2 

Briggs,  L.  J.,  and  McLane,  J.  W.,  The  Moisture  Equivalent  of  Soils; 
U.  S.  Dept.  Agr.  Bur.  Soils,  Bui.  45,  1907. 

1  An  ingenious  method  for  measuring  quantitatively  the  capillary  pull 
exerted  by  a  moist  soil  has  been  devised  by  Lynde  and  Dupre\  The 
apparatus  consists  of  a  glass  funnel  joined  to  a  thick- walled  capillary 
tube  by  means  of  a  piece  of  rubber  tubing,  a  water  seal  being  used  at 
this  point.  The  lower  end  dips  into  mercury.  The  soil  to  be  studied  is 
placed  in  the  funnel,  and  after  being  saturated  is  connected  by  means 
of  a  wick  of  cheesecloth  or  filter  paper  to  the  water  column  previously 
established  in  the  capillary  tube.  If  no  break  occurs  between  the  soil 
and  the  capillary  water  column,  the  apparatus  is  ready  for  use. 

The  excess  water  having  drained  away,  there  is  a  thinning  of  the  films 
on  the  soil  surface  due  to  evaporation.  Equilibrium  adjustments  now 
take  place,  which  result  in  the  drawing  upward  of  the  water  column. 
The  mercury  follows,  and  the  strength  of  the  pull  may  be  measured  by 
the  length  of  the  mercury  column.  The  old  method  of  measuring  capil- 
lary power  by  the  water  movement  through  a  dry  soil  is  vitiated  by  two 
conditions — the  length  of  time  necessary,  and  the  fact  that  the  maximum 
lift  cannot  be  obtained  due  to  excessive  friction.  This  new  method 
uses  a  wet  soil,  requires  only  a  short  time,  and  gives  a  more  nearly 
accurate  idea  of  the  power  of  the  capillary  pull.     It  does  not,  however, 


THE  FORMS  OF  SOIL-WATER 


169 


through  the  connecting  film  until  the  pull  at  A  and  that  at  B 
become  the  same.  Such  an  adjustment  might  go  on  over  a 
large  number  of  films,  and  if  one  end  of  the  column  was  ex- 
posed to  an  evaporation  of  just  the  correct  rate  and  the  other 
end  was  in  contact  with  plenty  of  moisture,  large  quantities 
of  water  would  be  moved  by  capillarity. 

This  capillary  movement  may  go  on  in  any  direction  in  the 
soil,  since  it  is  largely  independent  of  gravity;  yet  under 
natural  field  conditions  the  adjustment  tends  to  take  place 
very  largely  in  a  vertical  direction,  due  to  evaporation  and 
absorption  by  plants.  When  a  soil  is  exposed  to  evaporation, 
the  surface  films  are  thinned  and  water  moves  upward  to 
adjust  the  tension.  This  explains  why  such  large  quantities 
of  soil-water  may  be  lost  so  rapidly  from  an  exposed  soil. 
Capillary  adjustment  may  go  on  downward,  also,  as  is  the 
case  after  a  shower.  Here  the  rapidity  of  the  adjustment  is 
aided  by  gravity  and  movement  of  the  water  of  percolation. 

The  capillary  adjustment  in  a  soil  tends  to  take  place 
whether  or  not  the  soil  column  is  in  contact  with  free  water. 
If  no  gravity  water  is  present,  the  adjustment  is  merely  from 
a  moist  soil  to  a  drier  one.  In  studying  the  rate  and  height 
of  capillary  movement  of  water  in  any  soil,  especially  in  the 


yield  data  regarding  rate  of  movement, — a  factor  of  vital  importance 
to  plant  growth. 

Lynde  and  Dupre",  in  their  results,  confirm  the  statements  already  made 
regarding  the  relation  of  texture  to  capillary  power: 


Soil 

Diameter,  of 

Grains  in 
Millimeters 

Lift  of  Water 
Column,  in  Feet 

Medium  sand 

.50  -  .25 
.25  -  .10 
.10  -  .05 
.05  -  .005 
.005 

.98 

Fine  sand 

1.78 

Very  fine   sand 

4.05 

Silt 

9.99 

Clay  

26.80 

Lynde,   C.   J.,  and 
the  Capillary  Lift  in 
pp.  107-116,  1913. 

Dupre, 
,  Soils; 

H. 

JOL 

A.,  On  a  New   Me 
ir.   Amer.  Soc.  Agr 

thod  of  Measuring 
on.,  Vol.  5,  No.  2, 

170         NATURE  AND  PROPERTIES  OF  SOILS 

laboratory,  the  maintenance  of  a  supply  of  free  water  is 
usually  provided  for,  since  this  allows  a  nearer  approach  to 
the  maximum  capillary  capacity  for  any  point  in  the  column 
and  also  gives  the  most  rapid  capillary  adjustment. 

To  persons  familiar  with  the  habits  of  growing  plants,  it  is 
evident  that  capillary  movement  must  play  an  important 
part  in  their  nutrition,  since  the  rootlets  are  unable  to  bring 
their  absorptive  surfaces  in  contact  with  all  the  interstitial 
spaces  in  which  the  bulk  of  the  available  water  is  held.  Con- 
sequently a  consideration  of  the  movement  of  capillary  mois- 


FiG.  31. — Conventional  diagram  showing  the  mechanics  of  the  movement 
of  the  film  portion  of  the  capillary  water.  The  readjustment  takes 
place  in  the  direction  of  (A)  due  to  the  tension  developed  by  the 
greater  film  curvature  at  that  point. 

ture  is  necessary,  not  only  as  to  its  mechanics,-  but  also  in 
respect  to  the  factors  influencing  its  rate  and  height  of  move- 
ment. These  factors  are  as  follows:  (1)  surface  tension  and 
viscosity;  (2)  thickness  of  capillary  film;  (3)  texture;  and 
(4)  structure. 

Surface  tension  and  viscosity. — As  the  force  developed  by 
surface  tension  is  the  activating  factor  in  capillary  adjust- 
ment, any  change  in  the  former  will  influence  this  movement. 
Theoretically,  a  rise  in  temperature  or  the  presence  of  soluble 
salts  would  decrease  the  rapidity  of  the  capillary  activity  of 
soil-water.  In  a  normal  soil,  however,  the  change  of  surface 
tension  is  generally  not  sufficient  to  have  any  very  great  prac- 
tical influence.  Viscosity,  on  the  other  hand,  is  much  more 
important.    If  the  viscosity  of  water  at  0°  C.  is  taken  as  100, 


THE  FORMS  OF  SOIL-WATER  171 

its  viscosity  at  25°  is  50  and  at  30°,  45.  This  explains  to  a 
large  degree  the  increased  rate  of  capillary  movement  due  to 
temperature  rise.1  The  distance  of  such  adjustment  would, 
however,  be  lessened  somewhat.  Salts  in  solution  would  tend 
to  check  the  rate  of  capillary  movement  both  through  in- 
creased viscosity  and  the  influence  on  surface  tension.2  It 
would  only  be  in  alkali  soils,  where  the  concentration  of  soluble 
salts  is  very  great,  that  any  considerable  retardation  would 
occur. 

Thickness  of  capillary  film. — It  has  been  repeatedly  noticed, 
in  the  study  of  the  capillary  adjustment  between  two  soils 
that  the  lower  the  percentage  of  water,  the  slower  is  the  move- 
ment. This  indicates  that  the  thickness  of  the  outer  capillary 
film,  which  connects  the  interstices  in  which  lies  the  bulk  of 
the  movable  soil-water,  is  an  important  factor  in  the  rate  of 
movement. 

The  above  phenomena  may  be  empirically  explained  as  fol- 
lows :  Let  it  be  supposed  that  a  withdrawal  of  water  occurs  at 
A  (see  Fig.  32),  the  interstitial  space  between  two  of  the 
particles,  the  water  surface  being  represented  by  the  line  aa'. 
There  is  an  immediate  increase  in  the  curvature  of  this  sur- 
face, and  water  tends  to  flow  through  the  capillary  film  chan- 
nel   (ccV)   toward  this  area  of  greater  tension.     If  water 

1  Bouyoucos  has  shown  that  the  movement  in  a  soil  column  of  uniform 
moisture  is  from  the  warmer  portion  toward  the  colder.  The  movement 
from  a  moist  layer  to  a  dryer  one  goes  on  more  rapidly  than  when  the 
moist  soil  is  cool  and  the  dry  soil  warm.  Bouyoucos,  G.  J.,  Effect  of 
Temperature  on  Movement  of  Water  Vapor  and  Capillary  Moisture  in 
Soils;  Jour.  Agr.  Kes.,  Vol.  V,  No.  4,  pp.  141-172,  Oct.,  1915. 

aWollny,  E.,  Untersuchungen  uber  die  Kapillare  Leitung  des  Wassers 
in  Boden.  Forsch.  a.  d.  Gebiete  d.  Agr.-Physik,  Band  7,  Seite  269-308, 
1884.  Also,  Forsch.  a.  d.  Gebiete  d.  Agri.-Physik,  Band  8,  Seite  206-220. 
1885. 

Briggs,  L.  J.,  and  Lapham,  M.  H.,  Capillary  Studies;  U.  S.  Dept. 
Agr.  Bur.  Soils,  Bui.  19,  pry  5-18,  1902. 

Karraker,  P.  E.,  Effect  on  Soil  Moisture  of  Changes  in  the  Surface 
Tension  of  the  Soil  Solution  brought  about  by  the  Addition  of  Soluble 
Salts;  Jour.  Agr.  Ees.,  Vol.  IV,  No.  2,  pp.  187-192,  May,  1915. 

Davis,  K.  O.  E.,  The  Effect  of  Soluble  Salts  on  the  Physical  Proper- 
ties of  Soils;  U.  S.  Dept.  Agr.  Bur.  Soils,  Bui.  82,  pp.  23-31,  1911. 


172         NATURE  AND  PROPERTIES  OF  SOILS 

continues  to  be  withdrawn  at  A,  this  adjustment  goes  on  with 
considerable  ease  until  the  film  channel  (cc'c")  becomes  so 
thin  as  to  cause  its  surface  now  (bb'b")  to  approach  very 
closely  to  the  surface  of  the  soil  particle  and  the  inner  capil- 
lary water.  The  sluggishness  of  the  water  movement  becomes 
a  factor  at  this  point,  impeding  the  capillary  adjustment  to- 
ward A.  This  point  of  sluggish  capillary  movement  has  been 
designated  by  Widtsoe1  as  the  point  of  lento-capillarity,  and 


Fig.  32. — Conventional  diagram  for  the  explanation  of  the  effect  of  the 
thickness  of  water  film  about  the  soil  particles  and  their  colloidal 
complexes  on  the  ease  of  capillary  adjustment. 

is  expressed  in  percentage  based  on  the  dry  weight  of  the 
soil.  It  lies  near  the  transition  zone  between  the  inner  and 
outer  capillary  water. 

The  amount  of  capillary  water  delivered  at  any  one  point, 
therefore,  will  obviously  be  influenced  by  the  thickness  of  the 
film  and  may  consequently  be  taken  as  a  measure  of  rate  of 
adjustment.  A  short  soil  column  should  deliver  more  water 
than  a  longer  one,  due  to  the  thicker  films  at  the  surface  of 
the  former.  King,2  in  studying  the  evaporation  from  the  sur- 
faces of  sand  columns  of  different  lengths,  their  bases  being 
in  contact  with  free  water,  obtained  some  significant  data. 


1  Widtsoe,  J.  A.,  and  McLaughlin,  W.  W.,  The  Movement  of  Water  in 
Irrigated  Soils;  Utah  Agr.  Exp.  Sta.,  Bui.  115,  pp.  223-231,  1912. 

2  King,  F.  H.,  Principles  and  Conditions  of  the  Movements  of  Ground 
Water;  U.  S.  Geol.  Survey,  19th  Ann.  Kept.,  Part  II,  p.  92,  1897- 
1898. 

Also  Briggs,  L.  J.,  and  Lapham,  M.  H.,  Capillary  Studies;  U.  S. 
Dept.  Agr.  Bur.  Soils,  Bui.  19,  pp.  24-25,  1902. 


THE  FORMS  OF  SOIL- WATER  173 

He  found,  for  example,  that  a  six-inch  column  would  deliver 
six  times  more  water  to  its  surface  in  a  given  time  than  a 
thirty-inch  column  operating  under  the  same  conditions. 

In  air-dry  soil  it  is  obvious  that,  before  capillarity  may 
function,  a  continuous  film  must  be  present.  Such  a  condi- 
tion is  impossible  unless  some  of  the  more  active  capillary 
moisture  is  in  the  soil.  The  water  content  in  a  soil  must  often 
be  rather  high  before  capillarity  is  a  noticeable  phenomenon. 
This  condition  is  taken  advantage  of  in  the  use  of  soil-mulches, 
where  a  loose  dry  layer  of  soil  on  the  surface  may  check 
evaporation  by  impeding  capillary  rise.  The  presence  of  oily 
substances  on  the  soil  grains  may  also  be  of  some  importance 
in  this  respect. 

Texture. — In  soils  of  fine  texture  not  only  is  the  amount 
of  film  surface  exposed  greater  than  in  coarse  soils  but  the 
curvature  of  the  films  is  also  greater,  due  to  the  shorter  radii. 
The  effective  pressure  exerted  by  the  films  is  consequently 
much  higher  in  fine-grained  soil.  Both  the  greater  exposure 
of  surface  and  the  increased  pressure  serve  to  raise  the  fric- 
tion coefficient  and  retard  the  rate  of  flow.  The  finer  the 
texture  of  the  soil,  other  factors  being  equal,  the  slower  is 
the  movement  of  capillary  water.  Water  should,  therefore, 
rise  less  rapidly  from  a  water-table  through  a  column  of  clay 
than  through  a  sand  or  a  sandy  loam. 

The  distance  to  which  water  may  be  drawn  by  the  effective 
capillary  power  of  a  soil,  equilibrium  being  established,  de- 
pends on  the  number  of  interstitial  angles.  The  greater  the 
number  of  angles,  the  greater  is  the  total  pulling  power  of 
the  films.  As  a  silt  soil  contains  a  larger  number  of  such 
angles,  its  capillary  pull  is  greater  than  that  of  sand,  and  con- 
sequently the  ultimate  movement  would  be  of  greater  scope. 
The  finer  the  texture,  then,  the  slower  is  the  rate  of  capillary 
movement  but  the  greater  is  the  distance. 

The  relation  of  texture  to  rate  and  height  of  capillary  move- 
ment in  air-dry  soil  is  shown  by  the  following  unpublished 


174 


NATURE  AND  PROPERTIES  OF  SOILS 


data,  obtained  in  the  laboratory  of  the  Department  of  Soil 
Technology,  Cornell  University : 

Table  XXXII 

EFFECT  OF    MOISTURE  ON  RATE  AND   HEIGHT   OF  CAPILLARY  RISE 
FROM  A  WATER-TABLE  THROUGH  AIR-DRY  SOIL 


SOIL 

1 
Houe 

1 
Day 

2 

Days 

3 

Days 

4 
Days 

5 
Days 

Sandy  soil 

Clayey  soil 

Silt  loam 

Inches 
3.5 
.5 
2.5 

Inches 
5.0 
5.7 
14.5 

Inches 
5.9 
8.9 
20.6 

Inches 
6.8 
10.9 
24.2 

Inches 

6.8 

12.2 

26.2 

Inches 
6.9 
13.3 
27.4 

It  is  seen  that  the  movement  in  sand  is  rapid,  one-half  of 
the  total  rise  being  attained  in  one  hour.  The  maximum 
height  is  reached  in  about  three  days.  The  silt  loam  in  this 
case  seems  to  be  of  just  about  the  proper  textural  condition 
for  a  fairly  rapid  rise,  yet  it  exerts  enough  capillary  pull  to 
attain  a  good  distance  above  the  water-table.  The  friction 
in  the  clay  is  greater,  however,  and  this  results  in  a  slower 
rate. 

Structure  has  already  been  shown  to  affect  capillary  capac- 
ity by  its  influence  on  the  angle  interstices  and  the  closeness 
of  the  contacts.  Evidently,  therefore,  it  may  alter  both  the 
rate  and  the  height  of  capillary  rise.  The  loosening  of  a  clay 
soil  or  the  compacting  of  a  sandy  soil  will  lessen  the  effective 
film  friction,  while  at  the  same  time  it  may  strengthen  the 
capillary  pull  resulting  in  a  faster  and  a  higher  capillary  flow 
of  water.  What  may  be  the  best  structural  condition  of  any 
soil  in  which  this  result  is  realized  to  its  highest  degree  can 
not  be  predicted  exactly.  In  general,  however,  this  point  is 
approached  when  the  soil  is  in  the  best  physical  condition  for 
crop  growth.  Tillage  operations,  tile  drainage,  and  the  addi- 
tion of  lime  and  organic  matter  operate  toward  this  result  by 
their  granulating  tendencies;  while  rolling,  by  compacting  a 


THE  FORMS  OF  SOIL- WATER  175 

too  loose  surface,  may  accomplish  the  same  effect  but  by  an 
opposite  process. 

At  certain  seasons  of  the  year  capillarity  should  be  im- 
peded near  the  surface,  as  it  continually  carries  valuable 
water  upward  to  be  lost  by  evaporation.  This  movement  may 
be  checked  somewhat  by  producing  on  the  soil  surface,  by 
appropriate  tillage,  a  layer  of  dry,  loose  soil.  This  layer,  called 
a  soil-mulch,  resists  wetting  because  of  its  dryness,  while  at 
the  same  time  it  affords  but  little  surface  and  few  angle  inter- 
stices for  effective  capillary  pull.  Moisture  also  moves  very 
slowly  from  a  moist,  cool  soil  to  a  dry,  warm  one.1  Thus  it  is 
that  a  farmer,  in  order  to  meet  immediate  or  future  plant 
needs,  may  alter  and  control  capillary  movement  by  careful 
attention  to  physical  conditions,  especially  those  at  the  sur- 
face where  evaporation  is  always  active. 

93.  Gravitational  water  and  its  movement. — As  soon 
as  the  capillary  capacity  of  a  soil  column  is  satisfied,  further 
addition  of  moisture  will  cause  the  appearance  of  free  water 
in  the  air  spaces.  By  the  attraction  of  gravity,  this  water 
moves  forward  through  the  soil  at  a  rate  varying  with  con- 
ditions. In  general,  the  flow  is  governed  by  four  factors — 
pressure,  temperature,  texture,  and  structure.  An  under- 
standing of  the  operation  of  these  forces  is  important,  since 
the  rapid  elimination  of  free  water  from  the  soil  is  necessary 
for  normal  plant  growth. 

It  is  very  evident  that  any  pressure  exerted  on  a  water 
column  will  alter  the  rate  of  flow.  Under  normal  conditions 
pressure  may  arise  from  two  sources,  atmospheric  pressure 
and  the  weight  of  the  water  column.  Changes  in  barometric 
pressure  are  communicated  to  gravitational  water  through  a 
movement  of  the  soil-air.  As  the  mercury  column  rises  more 
air  is  forced  into  the  soil  and  the  pressure  on  the  soil-water 

1  Bouyoucos,  G.  Jv  Effect  of  Temperature  on  Movement  of  Water 
Vapor  and  Capillary  Moisture  in  Soils;  Jour.  Agr.  Kes.,  Vol.  V,  No.  4, 
pp.  141-172,  Oct.,  1915. 


176         NATURE  AND  PROPERTIES  OF  SOILS 

increases.  The  weight  of  the  free  water  column  may  also 
have  some  influence.  Although  King1  and  Welitschkowsky2 
have  shown  that  definite  relationships  exist  between  the  move- 
ment of  gravity  water  and  both  atmospheric  pressure  and 
weight  of  water  column,  the  practical  field  importance  of  these 
factors  are  rather  slight. 

A  rise  in  temperature  of  the  soil  not  only  varies  the  relative 
amounts  of  capillary  and  free  water  present,  but  at  the  same 
time  it  increases  the  fluidity  and  thus  facilitates  percolation. 
The  expansion  of  the  soil-air  also  tends  to  increase  such 
movement.  On  the  other  hand  the  swelling  of  hydrogels 
which  may  be  present  tends  to  impede  percolation  to  such  an 
extent  that  the  movement  of  free  water  through  a  heavy  soil 
is  often  markedly  checked  by  temperature  rise. 

Of  much  more  practical  importance  than  either  pressure 
or  temperature  in  the  flow  of  gravity  water  is  the  texture  and 
the  structure  of  the  soil.  In  working  with  sands  of  varying 
grades,  Welitschkowsky,3  Wollny,4  and  others  have  shown  that 
the  flow  of  water  varies  with  the  size  of  particle,  or  texture. 
King 5  has  demonstrated  that  in  general  the  rate  of  flow 
through  such  is  directly  proportional  to  the  square  of  the 
diameter  of  the  particles.    By  the  use  of  the  effective  mean 

1  King,  F.  H.,  Principles  and  Conditions  of  the  Movements  of  Ground 
Water;  U.  S.  Geol.  Survey,  19th  Ann.  Dep..,  Part  II,  pp.  67-206;  1897- 
1898. 

King,  F.  H.,  The  Soil,  p.  180,  New  York,  1906. 

^Welitschkowsky,  D.  von.,  Experiment elle  untersuchungen  uber  die 
Permeabilitat  des  Bodens  fur  Wasser;  Archiv.  f.  Hygiene,  Band  II, 
Seite  499-512.     1884. 

Wollny,  E.,  Untersuchungen  uber  die  Permeabilitat  des  Bodens  fur 
Wasser;  Forsch.  a.  d.  Gebiete  d.  Agr.-Physik,  Band  14,  Seite  1-28,  1891. 

3  Welitschkowsky,  D.  von.,  Experiment  elle  untersuchungen  uber  die 
Permeabilitat  des  Bodens  fur  Wasser;  Archiv.  f.  Hygiene,  Band  II, 
Seite  499-512,  1884. 

4  Wollny,  E.,  Untersuchunger  uber  den  Einfluss  der  Struktur  des 
Bodens  auf  dessen  Feuchtiglcetis — und  Temperaturverhaltnisse ;  Forsch. 
a.  d.  Gebiete  d.  Agr.-Physik,  Band  5,  Seite  167,  1882. 

5  King,  F.  H.,  Principles  and  Conditions  of  the  Movements  of  Ground 
Water;  U.  S.  Geol.  Survey,  19th  Ann.  Rep.,  Part  II,  pp.  222-224,  1897- 
1898. 


THE  FORMS  OF  SOIL-WATER  177 

diameter  of  a  sand  sample  he  was  able  to  calculate  a  theo- 
retical flow  which  compared  very  closely  to  observed  percola- 
tions. In  sandy  soils  low  in  organic  matter  this  law  holds 
in  a  very  general  way,  but  in  clays  it  fails  entirely.  For 
example,  if  such  a  law  was  in  force  a  sand  having  a  diameter 
of  .5  millimeter  would  exhibit  a  flow  10,000  times  greater 
than  that  through  a  clay  loam  with  a  diameter,  say,  of  .005 
millimeter;  whereas  the  actual  ratio,  as  observed  experimen- 
tally by  King,  was  less  than  200.  Such  a  discrepancy  is  to  be 
expected  as  it  is  impossible  accurately  to  apply  mathematics 
to  soils  carrying  any  appreciable  amount  of  colloidal  matter. 

Evidently,  therefore,  while  it  can  be  stated  as  a  general 
thesis  that  the  flow  of  gravity  water  varies  with  the  texture, 
being  much  more  rapid  through  a  coarse  than  through  a  fine 
soil,  no  law  can  be  deduced  for  soils,  since  structure 
exerts  such  a  modifying  influence.  The  percolation  in  a 
heavy  soil  takes  place  largely  through  lines  of  seepage,  which 
are  really  large  channels  developed  by  various  agencies. 
If  in  the  drainage  of  average  soil,  the  farmer  depended  on  the 
movement  of  water  through  the  individual  pore  spaces,  the 
soil  would  never  be  in  condition  for  crop  growth.  These  lines 
of  seepage  are  developed  by  the  ordinary  forces  that  function 
in  the  production  of  soil  granulation,  as  freezing  and  thawing, 
wetting  and  drying,  lime,  organic  matter,  roots,  and  tillage 
operations. 

94.  Determination  of  the  quantity  of  free  water  that 
a  soil  will  hold. — While  there  is  no  particular  advantage 
in  finding  the  quantity  of  gravitational  water  that  a  soil  will 
hold,  since  a  normal  soil  should  never  remain  saturated  for 
any  length  of  time,  it  is  nevertheless  of  interest  to  know  by 
what  means  such  data  may  be  obtained.  One  method  is  to 
saturate  a  soil  column  of  known  weight,  and  then,  by  exposing 
it  to  percolation,  measure  the  amount  of  water  that  is  lost. 
The  gravitational  water  can  then  be  expressed  in  terms  of  dry 
soil. 


178  NATURE  AND  PROPERTIES  OF  SOILS 

As  valuable  a  figure  may  be  obtained  by  calculation,  pro- 
viding the  specific  gravity  and  volume  weight  of  the  soil  is 
known  together  with  its  percentage  of  moisture  based  on  dry 
weight  when  it  is  capillarily  satisfied.  The  following  formu- 
lae1 may  be  used: 

1.  Percentage  pore  space  =  100  —  I x  — —  I 

Lsp.     gr.  1  J 

2.  Percentage  free  water  ==   %  ^"w^   ~~  %  water  at 

(based  on  dry  weight  Vo1-  wt-  maximum 

of  soil)  capillarity 

Suppose,  for  example,  that  a  sand  with  a  specific  gravity  of 
2.6  and  a  volume  weight  of  1.56  contains  20  per  cent,  of  water 
when  at  its  maximum  retentive  power.  Its  pore  space  would 
be  40  per  cent.  If  this  pore  space  were  filled  with  water,  the 
soil  would  contain  25.6  per  cent,  based  on  the  dry  weight  of 
the  soil  (per  cent,  pore  space  -^  vol.  wt.).  If  the  total  capac- 
ity of  the  soil  for  water  is  25.6  per  cent,  and  the  hygroscopic 
plus  the  capillary  capacity  is  20  per  cent.,  the  free  water  must 
be  5.6  per  cent.2 

95.  Importance  of  the  study  of  the  flow  and  composi- 
tion of  drainage  water. — A  clear  understanding  of  the 
factors  governing  the  flow  of  gravitational  water  is  of  special 
importance  in  tile  drainage  operations,  particularly  regarding 
the  depth  of  and  interval  between  tile  drains.  Since  percola- 
tion is  so  slow  in  a  heavy  soil  it  is  evident  that  the  tile  must 
be  near  the  surface  in  order  to  secure  efficient  drainage.  In 
a  sand  the  depth  may  be  increased,  because  of  the  slight  re- 

1  Percentage  of  pore  space  represents  the  percentage  of  water  by 
volume  that  would  occupy  such  a  space.  Percentage  of  water  by  volume 
divided  by  volume  weight  gives  percentage  of  water  based  on  dry  weight 
of  soil.  Conversely,  multiplying  percentage  of  moisture  calculated  on 
dry  weight  of  soil  by  volume  weight  will  give  percentage  of  water  by 
volume. 

The  air  space  in  a  soil  at  any  particular  moisture  content  may  be  cal- 
culated as  follows: 

Percentage  of  air  space  =  %  pore  space  —  (%H20  X  Vol.  Wt.) 

2  Below  will  be  found  some  generalized  moisture  data  on  two  distinct 


THE  FORMS  OF  SOIL- WATER  179 

sistance  offered  to  water  movement.  The  depths  for  laying  tile 
in  a  heavy  soil  range  from  one  and  a  half  to  two  and  a  half 
feet,  while  in  a  sand  the  tile  may  often  be  placed  as  deep  as 
four  feet  below  the  surface.  It  is  evident  also  that  the  less 
deep  a  tile  drain  is  laid  the  less  distance  on  either  side  it  will 
be  effective  in  removing  the  water;  consequently  on  a  clay 
soil  the  laterals  must  be  relatively  close  as  compared  to  the 
interval  generally  recommended  for  a  sandy  soil.  A  rational 
understanding  of  the  movements  of  gravitation  water  is 
clearly  necessary  in  the  installation  of  tile  drains  not  only 
that  the  system  may  be  efficient,  but  also  that  a  minimum 
effective  cost  may  be  realized.1 

The  water  lost  from  the  soil  by  drainage  is  of  especial  in- 
terest in  plant  production  because  of  the  large  amounts  of 
nutrient  elements  carried  away  each  year.  Such  loss  is  par- 
ticularly important  in  regard  to  the  lime  and  nitrogen.2  The 
equivalent  of  approximately  500  pounds  of  sodium  nitrate 
and  1000  pounds  of  calcium  carbonate  have  been  known  to 
leach  from  an  acre  of  bare  soil  every  year  under  humid  con- 
ditions. 

classes  of  soils.  As  usual,  all  of  the  moisture  data  is  expressed  as  per- 
centage based  on  absolutely  dry  soil. 

Sandy  Clayey 

Soil  Soil 

Specific   gravity    2.67  2.65 

Volume  weight   1.60  1.20 

Pore   space    40.0%  54.8% 

Hygro.    coefficient    1.0%  10.0% 

Optimum    moisture    (average) 10.0%  30.0% 

Maximum    field    capacity 17.0%  44.0% 

Air  space  at  hygro.  coefficient 38.4%  42.8% 

Air  space  at  opt.  moisture 24.0%  18.7% 

Air  space  at  max.  field  capacity 12.8%  1.9% 

Possible  free  water 8.0%  1.6% 

See  Kopecky,  J.,  Die  physikaliscJien  Eigenschaften  des  Boden; 
Internat.  Mitt  f.  Bodenkunde,  Bd.  IV,  Heft  2-3,  Seite  138-180.     1914. 

1  For  a  more  complete  discussion  of  tile  drains,  see  Chap.  X,  para- 
graph  110. 

aLyon,  T.  L.,  and  Bizzell,  J.  A.,  Lysimeter  Experiments;  Cornell 
Univ.  Agr.  Exp.  Sta.,  Memoir  12,  June,  1918. 


180         NATURE  AND  PROPERTIES  OF  SOILS 

Two  methods  of  procedure  are  available  for  the  study  of 
drainage  problems — the  use  of  an  efficient  system  of  tile 
drains,  and  the  construction  of  lysimeters.  For  the  first 
method  an  area  should  be  chosen  where  the  tile  drain  receives 
only  the  water  from  the  area  in  question  and  where  the  drain- 
age is  efficient.  A  study  of  the  amounts  of  flow  throughout 
a  term  of  years  will  yield  much  valuable  data  concerning  the 
factors  already  discussed.  An  analysis  of  the  drainage  water 
will  throw  light  on  the  ordinary  losses  of  plant  nutrients  from 
a  normal  soil  under  a  known  cropping  system.  The  advantage 
of  such  a  method  of  attack  lies  not  only  in  the  fact  that  a 
large  area  of  undisturbed  soil  is  considered,  but  also  in  the 
opportunity  to  study  practical  field  treatments  in  relation  to 
the  movement  and  composition  of  drainage  water. 

The  lysimeter  method,  however,  has  been  the  usual  mode  of 
approaching  such  problems.  In  this  method  a  small  block  of 
soil  is  used,  being  entirely  isolated  by  appropriate  means  from 
the  soil  surrounding  it.  Effective  and  thorough  drainage  is 
provided.  The  advantages  of  this  method  are  that  the  varia- 
tions in  a  large  field  are  avoided,  the  work  of  carrying  on  the 
study  is  not  so  great  as  in  a  large  field,  and  the  experiment 
is  more  easily  controlled.  One  of  the  best-known  sets  of  lysi- 
meters is  that  at  the  Rothamsted  Experiment  Station1  in  Eng- 
land. Here  blocks  of  soil  one  one-thousandth  of  an  acre  in 
surface  area  were  isolated  by  means  of  trenches  and  tunnels, 
and,  supported  in  the  meantime  by  perforated  iron  plates, 
were  permanently  separated  from  the  surrounding  soil  by 
masonry.  The  blocks  of  soil  were  twenty,  forty,  and  sixty 
inches  in  depth,  respectively.  Facilities  for  catching  the  drain- 
age were  provided  under  each  lysimeter.  The  advantages  of 
such  a  method  of  construction  lies  in  the  fact  that  the  struc- 
tural condition  of  the  soil  is  undisturbed  and  consequently  the 
data  are  immediately  trustworthy. 

^awes,  J.  B.,  Gilbert,  J.  H.,  and  Warington,  E.,  On  the  Amount 
and  Composition  of  the  Bain  and  Drainage  Waters  Collected  at  Bothar.i- 
sted;  Jour.  Roy.  Agr.  Soc,  Ser.  II,  Vol.  17,  pp.  269-271,  1881. 


THE  FORMS  OF  SOIL-WATER 


181 


At  Cornell  University1  a  series  of  cement  tanks  sunk  in 
the  ground  have  been  constructed.  Each  tank  is  about  four 
feet  and  two  inches  square  and  about  four  feet  deep.  A  slop- 
ing bottom  is  provided,  with  a  drainage  channel  opening  into 


^ji£M^t^±l^£^ii 


6-0 A  O 


Fig.  33. — Cross  section  of  the  lysimeter  tanks  at  Cornell  University, 
Ithaca,  New  York.  Each  tank  is  one  of  a  series,  one  tunnel  serving 
the  two  rows.  Dimensions  are  given  in  feet  and  inches.  Soils  under 
investigation  (a),  outlet  (p),  can  for  catching  drainage  water  (c) 
and  sky-light  (w). 


a  tunnel  beneath  and  at  one  side.  As  the  tanks  are  arranged 
in  two  parallel  rows,  one  tunnel  suffices  for  both.  (See  Fig. 
33.)     The  sides  of  the  tanks  are  treated  with  asphaltum  in 

1  Lyon,  T.  L.,  Tanks  for  Soil  Investigation  at  Cornell  University  ; 
Science,  N.  Ser.,  Vol.  29,  No.  746,  pp.  621-623,  1909. 

There  are  other  types  of  lysimeters.  See,  for  example,  Mooers,  C.  A., 
and  Maclntire,  W.  H.,  Two  Equipments  for  Investigation  of  Soil  Leach- 
ings:  I.  A  Pit  Equipment.  II.  A  Hillside  Equipment;  Tenn.  Agr.  Exp. 
Sta.,  Bui.   Ill,   1915. 

Maclntire,  W.  H.,  and  Mooers,  C.  A.,  A  Pitless  Lysimeter  Equip- 
ment; Soil  Sci.,  Vol.  XI,  No.  3,  pp.  207-209,  Mar.,  1921. 


182         NATURE  AND  PROPERTIES  OP  SOILS 

order  to  prevent  solution.  The  soil  must  of  course  be  placed 
in  the  tanks,  this  causing  a  disturbance  of  its  structural  con- 
dition. As  a  consequence,  data  as  to  rate  of  flow  and  com- 
position of  the  drainage  water  are  rather  unreliable  for  the 
first  few  years.  Such  an  experiment  must  necessarily  be  of 
considerable  duration. 

96.  Thermal  movement  of  water. — Little  has  been  said 
as  yet  regarding  this  mode  of  water  movement,  the  vapor 
flow,  which  is  not  peculiar  to  one  form  of  soil-water  but  affects 
them  all.  It  is  at  once  apparent  that  the  movement  of  water- 
vapor  can  be  of  little  importance  within  the  soil  itself,  since 
it  depends  so  largely  on  the  diffusion  and  convection  of  the 
soil-air.  While  the  soil-air  is  no  doubt  practically  always 
saturated  with  water-vapor,  the  loss  of  moisture  by  this  means 
is  slight.  Buckingham  x  has  shown  that,  while  sand  allows 
such  a  movement  to  the  greatest  degree,  the  loss  through  any 
appreciable  depth  of  layer  is  almost  negligible.  The  question 
of  the  thermal  movement  of  water  at  the  soil  surface,  however, 
is  vital  in  farming  operations.  At  this  point  the  moisture  is 
exposed  to  sun  and  wind,  and  drying  goes  on  rapidly,  the  free, 
capillary,  and  a  part  of  the  hygroscopic  water  vaporizing  in 
the  order  named.  If  the  loss  of  the  moisture  in  the  surface 
layer  of  soil  was  the  only  consideration,  the  problem  would 
not  be  serious;  but  the  movable  water  of  the  whole  soil  sec- 
tion must  be  considered  also.  As  the  films  at  the  surface  be- 
come thin,  a  capillary  movement  begins,  and  if  the  evapora- 
tion is  not  too  rapid  a  considerable  loss  of  water  may  occur  in 
a  short  time.  The  moisture  thus  lost  is  that  of  most  value 
to  plants.  The  evaporation  from  the  bare  soil  in  the  Rotham- 
sted  lysimeters2  averaged  about  seventeen  inches  a  year,  with 

1  Buckingham,  Ev  Studies  on  the  Movement  of  Soil  Moisture;  U.  S. 
Dept.  Agr.  Bur.  Soils,  Bui.  38,  pp.  9-18,  1907. 

See  also  Bouyoucos,  G.  J.,  Effect  of  Temperature  on  Movement  of 
Water  Vapor  and  Capillary  Moisture  in  Soils;  Jour.  Agr.  Ees.,  Vol.  V, 
No.  4,  pp.   141-172,  Oct.,  1915. 

2Warington,  E.,  Physical  Properties  of  the  Soil,  p.  109;  Clarendon 
Press,  Oxford,  1900. 


THE  FORMS  OF  SOIL-WATER  183 

a  rainfall  ranging  from  twenty-two  to  forty -two  inches.  This 
means  that  from  one-third  to  one-half  of  the  effective 
rainfall  was  entirely  lost  as  thermal  water.  The  necessity  of 
checking  such  a  loss  becomes  apparent,  especially  in  regions 

(where  rainfall  is  slight  or  drought  periods  are  likely  to  occur. 
As  no  country  is  free  from  one  or  the  other  of  such  con- 
tingencies, the  great  prominence  that  methods  of  moisture 
conservation  hold  in  systems  of  soil  management  is  under- 
standable. While  means  of  checking  losses  by  leaching  and 
run-off  are  advocated,  effective  retardation  of  surface  evapora- 
tion is  always  emphasized. 


CHAPTER  IX 

THE  WATER  OF  THE  SOIL  IN  ITS  RELATION  TO 
PLANTS 

Water  begins  its  service  to  plants  by  promoting  the  proc- 
esses of  soil  weathering,  which  results  in  the  simplification  of 
compounds  for  plant  utilization.  It  also  functions  more  di- 
rectly in  plant  development  in  maintaining  the  turgidity  of 
the  cells,  in  carrying  materials,  regulating  temperature  and 
in  furnishing  a  supply  of  hydrogen  and  oxygen  for  the  plant. 
These  direct  and  indirect  functions  of  water  in  relation  to 
plant  growth  may  be  considered  from  a  number  of  different 
viewpoints. 

97.  Functions  of  water  to  plants. — Water  acts  as  a 
solvent  and  as  a  medium  for  the  transfer  of  nutrients  from 
the  soil  to  the  plant.  This  transfer  relationship  is  rather 
complex,  since  most  nutrient  materials  penetrate  the  cell- walls 
of  the  absorbing  surfaces  of  the  roots  in  an  ionic  condition. 
As  a  nutrient  water  becomes  a  part  of  the  cell  contents  with- 
out change  or  is  broken  down  into  its  elements  and  utilized  in 
the  production  of  new  compounds.  In  addition,  water  by 
maintaining  turgidity,  in  equalizing  the  temperature  by  evap- 
oration from  the  leaves,  and  in  facilitating  quick  shifts  of 
nutrients  and  food  from  one  part  of  the  plant  to  another, 
acts  as  a  carrier  during  assimilation  and  while  synthetic  and 
metabolic  processes  are  going  on. 

Soil-moisture,  therefore,  in  proper  amounts,  becomes  one 
of  the  controlling  factors  in  crop  growth  and  must  be  looked 
to  before  the  maximum  utilization  of  the  nutrient  elements 
can  be  expected.    The  amount  of  water  held  within  the  plant 

184 


WATER  OF  SOIL  IN  ITS  RELATION  TO  PLANTS    185 

is  not  large,  however,  in  comparison  with  the  amount  lost 
by  transpiration,  although  green  plants  contain  from  60  to 
90  per  cent,  of  moisture. 

Because  of  the  readiness  with  which  moisture  passes  from 
plants  into  the  atmosphere,  large  quantities  must  be  taken 


Fig.  34. — The  effect  of  increasing  water  supply  on  the  production  of  dry- 
matter   in   various  crops.     The   water   is  expressed   in   acre-inches. 


10 

IO 

a 

6 

A 
2 

COIZN 

VVHETAT 

—~POTt 

ZTOES. 

o 

i 

O                 2 

o             3 

o             A 

o             & 

L0             <3 

O  JN.  M20. 

from  the  soil  in  order  that  the  plant  may  maintain  its  proper 
turgor.  That  the  crop  may  be  properly  supplied  with  water, 
optimum  moisture  conditions  should  prevail  in  the  soil  at 
all  times  during  the  growing  season.  It  must  not  be  inferred 
that  loss  through  the  plant  is  the  only  means  by  which  mois- 
ture leaves  the  soil,  since  drainage  and  evaporation  are  by 
no  means  insignificant  factors. 


186 


NATURE  AND  PROPERTIES  OF  SOILS 


98.  Influence  of  water  on  the  plant.1 — As  the  amount 
of  water  available  to  a  crop  is  increased  up  to  a  certain  point, 
the  vegetative  growth  also  is  usually  increased,  the  plant  be- 
coming more  succulent.  The  percentage  of  moisture  in  the 
crop,  even  at  harvest  time,  is  usually  high.  Shipping  qualities 
are  depressed  with  increased  moisture,  especially  if  the  water 
available  is  excessive.  With  an  enlargement  of  the  plant 
cell  a  change  probably  occurs  in  the  cell  contents,  tending 
toward  a  greater  susceptibility  to  disease. 

Ripening  especially  is  delayed  by  large  amounts  of  mois- 
ture, tillering  is  diminished,  and  the  percentage  of  protein 
content  of  the  crop  is  decreased.  It  is  a  curious  fact  that 
many  of  the  general  and  morphological  effects  of  large  quan- 
tities of  available  water  on  plant  growth  are  the  same  as  those 
caused  by  the  presence  of  too  much  soluble  nitrogen.  In 
cereals  the  stimulation  from  a  large  supply  of  water  is 
shown  especially  in  the  ratio  of  grain  to  straw.  Widt- 
soe  's  2  findings  in  this  regard  are  representative  of  the  data  3 
available  on  this  point: 

Table  XXXIII 

DISTRIBUTION  OF  DRY  MATTER  BETWEEN  GRAIN  AND  STRAW  WITH 
VARYING  AMOUNTS  OF  WATER. 


Inches  of  water  ap- 
plied   

Grain  in  percentage 
of  dry  matter  of 
entire  crop 


5 
44 


7% 
43 


10 


43 


15 


41 


25 


38 


35 


37 


50 


33 


1  Mitscherlich,  E.  A.,  Das  Wasser  als  Vegetations f  aktor ;  Landw.  Jahr., 
Band  42,  Seite  701-717,  1912. 

aWidtsoe,  J.  A.,  The  Production  of  Dry  Matter  with  Different  Quan- 
tities of  Irrigation  Water;  Utah  Agr.  Exp.  Sta.,  Bui.  116,  p.  49,  1912. 

8  Bunger,  H.,  tJber  den  Einfluss  Verschieden  Eohen  Wasser gehalts  des 
Bodens  in  den  Einzelhen  Vegetationsstadien  bei  Verschiedenem  Nahr- 
stoffreichtum  auf  die  EntwicTclung  des  Haferpflanzen;  Landw.  Jahrb., 
Band  35,  Seite  941-1051,   1906. 

Also,  Seelhorst,  C,  von,  und  Freckmann,  W.,  Der  Einfluss  des  Was- 
sergehaltes  des  Bodens  auf  die  Ernten  und  die  Ausbilding  V erschiedener 
Getriedevarietaten ;  Jour.  f.  Landw.,  Band  51,  Seite  253-269,  1903. 


WATER  OF  SOIL  IN  ITS  RELATION  TO  PLANTS    187 

As  a  rule,  this  depression  of  the  ratio  of  grain  to  straw  is 
not  due  to  an  actual  decrease  in  the  grain,  but  to  a  corre- 
spondingly greater  production  of  dry  matter  in  the  vege- 
tative parts.  As  available  water  is  augmented,  the  dry  mat- 
ter of  plants  increases  until  a  maximum  is  reached.  The  gen- 
eral relationships  are  well  exemplified  by  data  from  Widtsoe  x 
(Fig.  34),  although  other  equally  valuable  figures  may  be 
obtained  from  von  Seelhorst 2  and  Atterberg,3  who  have  done 
much  work  on  the  subject. 

99.  The  water  requirements  of  plants. — As  might  be 
expected,  the  pounds  of  water  transpired  for  every  pound  of 
dry  matter  produced  in  the  crop  is  very  large.  This  figure, 
called  the  transpiration  ratio,  or  water  requirement,  ranges 
from  200  to  500  for  crops  in  humid  regions,  and  almost  twice 
as  much  for  crops  in  arid  climates.  An  accurate  determina- 
tion of  the  transpiration  ratio  of  a  crop  is  somewhat  difficult, 
due  to  the  methods  of  procedure  necessary  and  also  to  the 
difficulty  of  controlling  the  numerous  factors  that  influence 
the  transpiration.  For  really  reliable  figures  the  plants  must 
be  grown  in  cans  or  pots  in  order  that  the  water  lost  may 
be  determined  accurately  by  weighing.  If  there  is  no  percola- 
tion the  water  ordinarily  lost  from  a  cropped  soil  includes 
both  that  evaporated  from  the  soil  surface  and  that  tran- 
spired from  the  leaves.  The  former  loss  may  be  controlled 
largely  in  one  of  two  ways:  (1)  by  covering  the  soil  so  that 
evaporation  is  absolutely  checked  and  the  only  loss  is  by 
transpiration;  or  (2)  by  determining  the  evaporation  from  a 
bare  pot  and,  by  substracting  this  from  the  total  water  loss 

1  Widtsoe,  J.  Av  The  Production  of  Dry  Matter  with  Different  Quan- 
tities of  Irrigation  Water;  Utah  Agr.  Exp.  Sta.,  Bui.  116,  pp.  19-25, 
1912. 

3  Seelhorst,  C,  von,  und  Krzymowski,  E.,  Versuch  uiber  den  Einfluss, 
welchen  das  Wasser  in  dem  Verschiedenem  Vegetationsstadien  des  Hafers 
auf  sein  Wachstum  ausubt;  Jour.  f.  Landw.,  Band  53,  Seite  357-370, 
1905. 

'Atterberg,  A.,  Die  Variationem  der  Nahrstoffgehalte  oei  dem  Hafer; 
Jour.  f.  Landw.,  Band  49,  Seite  97-113,  1901. 


188         NATURE  AND  PROPERTIES  OF  SOILS 

from  a  cropped  soil,  finding  the  loss  due  to  transpiration 
alone. 

An  objection  to  the  former  method  is  that  any  covering 
which  interferes  with  evaporation  interferes  with  proper  soil 
aeration  also  and  may  render  soil  conditions  abnormal.  In 
the  second  method,  however,  an  even  more  serious  error  en- 
ters, since  the  evaporation  from  the  bare  soil  is  not  the  same 
as  that  from  a  soil  covered  by  vegetation  because  of  the  effect 
of  shading.  Moreover,  due  to  the  action  of  the  roots,  less 
water  is  likely  to  move  to  the  surface  by  capillary  attraction 
in  the  cropped  soil.  Therefore  any  data  that  may  be  quoted 
can  be  only  general  in  its  application,  not  only  because  of  the 
errors  of  determination  but  also  because  of  the  great  num- 
ber of  factors  that  under  normal  conditions  may  vary  the 
transpiration  ratio.  The  following  data  drawn  from  various 
investigators  working  by  the  general  methods1  already  out- 
lined, give  some  idea  of  the  water  transpired  by  different 
crops,  due  allowance  being  made  for  various  disturbing  fac- 
tors.    (See  Table  XXXIV,  page  189.) 

100.  Factors  affecting  transportation.2 — It  is  obvious 
from  the  figures  quoted  that  the  transpiration  ratio  of  a  crop 
is  the  resultant  of  a  number  of  influences.3  The  factors  may 
be  listed  under  three  heads,  as  follows : 

1.  Crop. — Difference  due  to  different  crops  and  to  vari- 
ations of  the  same  crop. 

1 A  brief  discussion  of  the  various  methods  is  found  as  follows : 
Montgomery,    E.    G.,    Methods    of   Determining    the    Water    'Require- 
ments of  Crops;  Proc.  Amer.  Soc.  Agron.,  Vol.  3,  pp.  261-283,  1911. 

Also  Briggs,  L.  J.,  and  Schantz,  H.  L.,  The  Water  Requirement  of 
Plants;  U.  S.  Dept.  Agr.,  Bur.  Plant  Ind.,  Bui.  285,  1913. 

2  Kiesselbach,  T.  A.,  Transpiration  as  a  Factor  in  Crop  Production; 
Nebr.  Agr.  Exp.  Sta.,  Res.  Bui.  6,  June,  1916. 

3  A  complete  review  of  the  literature  concerning  the  climatic  and 
soil  factors  in  their  effect  on  transpiration  may  be  found  as  follows: 
Briggs,  L.  J.,  and  Shantz,  H.  L.,  The  Water  Requirement  of  Plants; 
U.  S.  Dept.  Agr.,  Bur.  Plant  Ind.,  Bui.  285,  1913. 

See  also,  Briggs,  L.  J.,  and  Shantz,  H.  L.,  Daily  Transpiration  dur- 
ing the  Normal  Growth  Period  and  its  Correlation  with  the  Weather; 
Jour.  Agr.  Ees.,  Vol.  VII,  No.  4,  pp.  155,  212,  Oct.,  1916. 


WATER  OF  SOIL  IN  ITS  RELATION  TO  PLANTS    189 


Table  XXXIV 

WATER    REQUIREMENTS    OF    PLANTS    AS    DETERMINED    BY   DIFFER- 
ENT  INVESTIGATORS. 


Crop 

La  wes  1 
Harpen- 

DEN, 

England, 
1850 

Wollny  2 

Munich, 

Germany 

1876 

Hell- 

RIEGEL  a 

Dahme, 

Germany 

1883 

King4 
Madison, 
Wis.,  1895 

Leather  5 
Pusa, 
India 
1911 

Briggs 

and 

Shantz* 

Akron, 

Colo. 

1911-1913 

Barley   . . . 

Beans 

Buckwheat 

Clover    .  . . 

Maize 

Millet 

Oats   

Peas 

Potatoes  .  . 
Rape 

Rye    

Wheat    ... 

258 
^209 

269 
259 
247 

774 

646 

>233 
447 
665 
416 

912 

310 

282 
363 
310 

376 
/273 

353 

338 

464 

576. 
y271 

503 

477 
385 

468 

/337 

469 
563 

544 

534 
736 

578 

m 

z-368 

/310 

597 

iaa 

636 
441 
685 
513 

1  Lawes,  J.  B.,  Experimental  Investigation  into  the  Amount  of  Water 
Given  off  by  Plants  during  their  Growth;  Jour.  Hort.  Soc,  London, 
Vol.  5,  pp.  38-63,  1850.  Pots  holding  42  pounds  of  field  soil  were  used. 
Evaporation  from  soil  was  reduced  to  a  very  low  degree  by  perforated 
glass  covers  cemented  on  the  pots.  The  figures  quoted  are  from  un- 
fertilized soil. 

8  Wollny,  E.,  Der  Einfluss  der  Pflamendecke  und  Beschattung  auf  die 
Physikalischen  Eigenschaften  und  die  Fruchtbarkeit  des  Bodens,  Seite 
125;  Berlin,  1877.  Wollny  grew  plants  in  sand  in  amounts  ranging 
from  5  to  12  kilograms.  Evaporation  was  reduced  to  a  very  low 
degree  by  perforated  covers.  Actual  evaporation  from  uncropped  cans 
was  observed,  however. 

3  Hellriegel,  H.,  Beitrage  zur  den  Naturwissensehaftlichen  Grundlagen 
des  Ackerbaus,  Seite  663;  Braunschweig,  1883.  Hellriegel  grew  plants 
in  4  kilograms  of  clean  quartz  sand  and  supplied  them  with  nutrient 
solutions.  The  loss  by  evaporation  from  uncropped  pots  was  used  in 
determining  losses  by  transpiration.  In  later  experiments  covers  were 
used  in  order  to  cut  down  evaporation. 

4  King,  F.  H.,  Physics  of  Agriculture,  p.  139;  published  by  author, 
Madison,  Wis.,  1910.  Also,  The  Number  of  Inches  of  Water  Required 
for  a  Ton  of  Dry  Matter  in  Wisconsin;  Wis.  Agr.  Exp.  Sta.,  11th  Ann. 
Rep.,  pp.  240-248,  1894;  and  The  Importance  of  the  Right  Amount  and 
Right  Distribution  of  Water  in  Crop  Production;  Wis.  Agr.  Exp.  Sta., 


190        NATURE  AND  PROPERTIES  OF  SOILS 

2.  Climate — Rain,  humidity,  sunshine,  temperature,  and 
wind. 

3.  Moisture  and  fertility.1 

Not  only  do  different  plants  2  show  a  variation  of  transpira- 
tion the  same  season,  but  the  same  plant  may  give  a  totally 
different  transpiration  in  separate  years.  This  is  due  in  part 
to  inherent  differences  in  the  plant  itself.  For  example,  the 
extent  of  leaf  surface  or  root  zone  would  materially  influence 
the  transpiration  relationship  under  any  given  condition. 
However,  a  great  deal  of  the  variation  observed  in  the  ratios 
already  quoted  arises  from  differences  in  climatic  conditions. 
As  a  general  thing,  the  greater  the  rainfall  the  higher  is 
the  humidity  and  the  lower  is  the  relative  transpiration. 
This  accounts  for  the  high  figures  obtained  by  Widtsoe  8  in 
Utah.     Montgomery4  found,  in  studying  the  water  require- 

1  Fertility  is  used  here  in  the  sense  of  potential  productivity.  It 
refers  especially  to  the  ultimately  available  nutrients  of  the  soil. 

2  Miller,  E.  C,  and  Coffman,  W.  B.,  Comparative  Transpiration  of 
Corn  and  the  Sorghums;  Jour.  Agr.  Res.,  Vol.  XIII,  No.  11,  pp.  579- 
604,  June,  1918. 

8  Widtsoe,  J.  A.,  The  Production  of  Dry  Matter  with  Different  Quan- 
tities of  Irrigation  Water;  Utah  Agr.  Exp.  Sta.,  Bui.  116,  1912.  Also, 
Irrigation  Investigations.  Factors  Influencing  Evaporation  and  Trans- 
piration; Utah  Agr.  Exp*  Sta.,  Bui.  105,  1909. 

4  Montgomery,  E.  G.,  and  Kiesselbach,  T.  A.,  Studies  in  Water  Re- 
quirements of  Corn;  Nebr.  Agr.  Exp.  Sta.,  Bui.  128,  p.  4,  1912. 

14th  Ann.  Rep.,  pp.  217-231,  1897.  King  used  cans  holding  about  400 
pounds  of  soil.  Some  were  set  down  into  the  earth  while  others  were 
not.  Part  of  the  work  was  carried  on  in  the  field;  the  remainder  was 
run  in  vegetative  houses.  Normal  soils  were  used.  Evaporation  from 
soil  was  very  low,  water  being  added  from  beneath.  The  data  quoted 
are  the  average  of  a  large  number  of  tests. 

8  Leather,  J.  W.,  Water  Requirements  of  Crops  in  India;  Memoirs, 
Dept.  Agr.,  India,  Chem.  Series,  Vol.  I,  No.  8,  pp.  133-184,  1910,  and 
No.  10,  pp.  205-281,  1911.  Jars  containing  from  12  to  48  kilograms 
of  soil  were  used.  Loss  by  evaporation  was  determined  on  bare  pots. 
The  plants  were  grown  in  culture  houses  or  in  screened  inclosures. 

6Briggs,  L.  J.,  and  Schantz,  H.  L.,  Relative  Water  Requirement  of 
Plants;  Jour.  Agr.  Research,  Vol.  Ill,  No.  1,  pp.  1-63,  1914.  Also, 
The  Water  Requirements  of  Plants;  U.  S.  Dept.  Agr.,  Bur.  Plant  Ind., 
Bui.  284,  1913.  Plants  were  grown  in  cans  holding  250  pounds  of  soil. 
Evaporation  from  soil  was  prevented  by  means  of  a  paraffin  covering. 
Work  was  conducted  in  screened  inclosures.  The  data  are  the  average 
of  several  years'  work. 


WATER  OF  SOIL  IN  ITS  RELATION  TO  PLANTS    191 

ments  of  corn  under  greenhouse  conditions,  that  an  increase 
in  the  percentage  humidity  from  42  to  65  lowered  the 
transpiration  ratio  from  340  to  191.  In  general,  temperature, 
sunshine,  and  wind  vary  together  in  their  effect  on  transpira- 
tion. That  is,  the  more  intense  the  sunshine,  the  higher  is 
the  temperature,  the  lower  is  the  humidity,  and  the  greater 
is  likely  to  be  the  wind  velocity.  All  this  would  tend  to  raise 
the  transpiration  ratio. 

From  the  soil  standpoint,  however,  the  factors  inherent 
in  the  soil  itself  are  of  more  vital  importance  as  regards  tran- 
spiration, since  they  can  be  controlled  to  a  certain  extent  un- 
der field  conditions.  An  increase  in  the  moisture  content  of  a 
soil  usually  results  in  an  increased  transpiration  ratio.  The 
work  of  Hellriegel 1  with  barley  grown  in  quartz  sand  con- 
taining a  nutrient  solution  may  be  cited  in  this  regard,  to- 
gether with  the  data  obtained  by  Montgomery  2  at  Lincoln, 
Nebraska,  with  maize  grown  in  a  loam  soil : 

Table  XXXV 

EFFECT  OF  SOIL-MOISTURE  ON  TRANSPIRATION. 


Barley — Hellriegel 

Maize — Montgomery 

SOIL-MOISTURE 

PERCENTAGE 

OF  TOTAL 

CAPACITY 

TRANSPIRATION 
RATIO 

SOIL-MOISTURE 

PERCENTAGE  OF 

TOTAL  CAPACITY 

TRANSPIRATION 
RATIO 

80 

60 
40 
30 
20 
10 

277 
240 
216 
223 
168 
180 

100 
80 
60 
45 
35 

290 
262 
239 
229 
252 

1  Hellriegel,  H.,  Beitrdge  zu  den  Naturwissenschaftlichen   Grundlage 
des  Aclcerbaus,  Seite  629,  Braunschweig,  1883. 

2  Montgomery,   E.    G.,   Methods   of  Determining #  the    Water  Require- 
ments of  Crops;  Proc.  Amer.  Soc.  Agron.,  Vol.  3,  p.  276,  1911. 


192 


NATURE  AND  PROPERTIES  OF  SOILS 


These  data  show  clearly  that  an  excessive  amount  of  mois- 
ture in  the  soil  is  not  a  favorable  condition  for  the  economical 
use  of  water. 

The  amount  of  available  nutrients  is  also  concerned  in  the 
economic  utilization  of  water.  In  general  the  data  along 
these  lines  show  that  the  more  productive  the  soil  the  lower 
is  the  transpiration  ratio.  Therefore,  a  farmer,  in  raising 
the  productivity  of  his  soil  by  drainage,  lime,  good  tillage, 
green-manures,  barnyard  manures,  and  fertilize^  provides 
at  the  same  time  for  a  greater  amount  of  plant  production 
for  every  unit  of  water  utilized.  The  total  quantity  of  water 
taken  from  the  soil,  however,  will  probably  be  larger. 

The  following  figures  from  Montgomery  x  are  representative 
of  data  available  on  this  phase : 


Table  XXXVI 

RELATIVE  WATER  REQUIREMENT  OF   MAIZE  ON   DIFFERENT  TYPES 
OF  NEBRASKA  SOILS,  1911. 


Soil 

Dry  Weight  op  Plants 
in  Grams  per  Pot 

Transpiration  Eatio 

MANURED 

UNMANURED 

MANURED 

UNMANURED 

Poor  (15  bushels) . . . 
Medium   (30  bushels) 
Fertile   (50  bushels). 

376 
413 
472 

113 
184 
270 

350 
341 
346 

549 
479 
392 

The  effects  of  texture  have  been  investigated  by  a  number 
of  men,  the  work  of  von  Seelhorst 2  and  of  Widtsoe  3  being 

1  Montgomery,  E.  G.,  Water  Requirements  of  Qorn;  Nebr.  Agr.  Exp. 
Sta.,  25th  Ann.  Kep.,  p.  xi,  1912. 

See  also,  Hellriegel,  H.,  Beitrage  zu  den  Naturwissenschaftlichen 
Grundlage  des  Ackerbaus,  Seite  629,  Braunschweig,  1883. 

2  Seelhorst,  C,  von.,  tJber  den  Wasserverbrauch  von  Boggen,  Gerste, 
Weizen,  und  Kartoffeln;  Jour.  f.  Landwirtschaft,  Band  54,  Heft  4, 
Seite  316-342,  1906. 

3  Widtsoe,  J.  A.,  Irrigation  Investigations.  Factors  Influencing  Evapo- 
ration and  Transportation;  Utah  Agr.  Exp.  Sta.,  Bui.  105,  1909. 


WATER  OF  SOIL  IN  ITS  RELATION  TO  PLANTS    193 

perhaps  the  most  reliable.  While  these  investigators  found 
in  general  that  plants  on  heavy  soils  exhibited  a  low  transpira- 
tion ratio,  hasty  conclusions  must  not  be  drawn.  Since  the 
fine-textured  soils  contain  more  nutrient  materials,  it  is  prob- 
able that  this  is  also  a  factor. 

101.  Amounts  of  water  necessary  to  mature  a  crop. — 
Although  it  may  be  seen  from  the  transpiration  ratios  cited 
that  the  amount  of  water  necessary  to  mature  the  average 
crop  is  very  large,  a  concrete  example  under  humid  condi- 
tions may  be  cited  to  advantage.  A  fair  estimate  of  the  dry 
matter  produced  in  the  above-ground  parts  of  a  forty-bushel 
crop  of  wheat  would  be  about  two  tons.  Assuming  the  tran- 
spiration ratio  to  be  300,  the  amount  of  water  actually  used 
by  the  plant  would  amount  to  600  tons  to  the  acre,  or  about 
5.2  inches  of  rainfall.  This  does  not  include  the  evaporation 
that  is  continually  going  on  from  the  soil  surface,  which  might 
very  easily  amount  to  as  much  more.  The  demand  in  total, 
to  say  nothing  of  run-off  and  drainage,  is  at  least  equal  to 
10  inches  of  rainfall. 

102.  Role  of  capillarity  in  supplying  the  plant  with 
water. — A  query  arises  at  this  point  regarding  the  mode 
by  which  this  immense  quantity  of  water  is  supplied  to  the 
plant.  The  rootlets,  especially  their  absorbing  surfaces,  are 
few  in  number  as  compared  with  the  interstitial  angles  that 
contain  most  of  the  water  retained  in  the  soil.  How,  then, 
does  the  plant  avail  itself  of  water  not  in  immediate  contact 
with  its  rootlets?  This  question  has  been  anticipated  in  the 
discussion  concerning  the  capillary  equilibrium  which  tends 
to  occur  in  all  soils.  As  soon  as  the  rootlet  begins  to  absorb 
at  one  point  the  film  in  that  interstitial  angle  is  thinned. 
A  considerable  convexity  of  the  water  surface  occurs  at  that 
point,  resulting  in  an  inward  pull,  which  causes  the  water  to 
move  in  all  directions  toward  that  point.  Thus  a  feeding 
rootlet  by  absorbing  some  of  the  moisture  with  which  it  is  in 
contact,  creates  a  condition  of  instability  which  results  in 


194         NATURE  AND  PROPERTIES  OF  SOILS 

considerable  film  movement.  It  can,  therefore,  be  said  that 
capillarity  is  an  important  factor  in  any  soil  in  supplying 
the  plant  with  proper  quantities  of  moisture. 

Many  of  the  early  investigators  have  over-estimated  the 
distances  through  which  this  adjustment  may  be  effective  in 
properly  supplying  the  plant.  It  must  always  be  kept  clearly 
in  mind  that  it  is  the  rate  of  water  supply  that  is  the  con- 
trolling factor.  Therefore,  capillarity,  although  it  may  act 
through  a  distance  of  eight  or  ten  feet  if  time  enough  be  al- 
lowed, may  actually  be  of  immediate  practical  importance 
through  only  a  few  inches  as  far  as  the  crop  is  concerned.  No 
extended  data  are  available  as  to  this  particular  phase,  but  the 
knowledge  of  capillary  movement  indicates  that  capillarity  of 
the  soil  is  of  greatest  importance  in  a  restricted  zone  immedi- 
ately around  the  surface  of  each  absorbing  root.1 

103.  Why  plants  wilt. — As  has  already  been  indicated, 
water  may  be  of  little  use  to  a  plant  because  of  distance,  since 
capillary  action  may  not  move  the  water  rapidly  enough 
for  normal  needs.  Water  near  at  hand  may  be  unavailable 
through  the  obstruction  of  capillarity,  friction  in  this  case 
being  the  cause.  As  the  rootlet  thins  the  interstitial  film  at 
any  point,  the  surface  tension  equilibrium  is  disturbed  and 
water  moves  toward  the  absorbing  surface.  This  movement  is 
rapid  enough  for  plant  needs  until  the  film  channels  on  the 
particles  become  thin.  As  such  a  condition  approaches,  fric- 
tion increases  rapidly,  cutting  down  the  capillary  movement 
to  such  an  extent  as  to  interfere  with  the  normal  functions 
of  the  plant. 

Wilting  occurs,  therefore,  merely  because  the  soil  is  unable 
to  move  the  water  rapidly  enough  for  crop  needs.  As  the 
friction  increases  very  rapidly  after  the  point  of  lento-capil- 
larity  is  reached,  the  wilting  coefficient  is  a  figure  somewhat 

xBurr,  W.  W.,  The  Storage  and  Use  of  Soil  Moisture;  Nebr.  Agr.  Exp. 
Sta.,  Ees.  Bui.  5,  1914. 

Miller,  E.  C,  Comparative  Study  of  the  Boot  System  and  Leaf 
Areas  of  Corn  and  Sorghums;  Jour.  Agr.  Ees.,  Vol.  VI,  No.  9,  pp.  311- 
331,  1916. 


WATER  OF  SOIL  IN  ITS  RELATION  TO  PLANTS    195 

less  than  the  percentage  representing  the  lento-capillarity. 
Since  the  inner  capillary  water  moves  very  sluggishly  if  at 
all,  wilting  must  occur  before  the  plant  has  drawn  to  any  great 
extent  on  this  part  of  the  capillary  moisture.  The  hygroscopic 
water  is,  therefore,  wholly  unavailable  to  plants  and  generally 
some  of  the  capillary  as  well,  although  Alway  1  has  shown  that 
under  certain  conditions  the  plant  may  reduce  the  moisture 
down  to  the  hygroscopic  coefficient.  The  wilting  coefficient  ex- 
pressed in  soil-moisture  terms  may  be  located  somewhere  be- 
tween the  hygroscopic  coefficient  and  the  point  of  lento- 
capillarity. 

104.  The  wilting  coefficient  and  its  determination. — It 
has  been  known  for  many  years  that  the  common  plants  pos- 
sess different  capacities  for  resisting  drought.  This  has  usu- 
ally been  ascribed  to  one  or  more  of  three  causes:  (1)  differ- 
ences in  root  extension;  (2)  differences  in  ability  to  become 
adjusted  to  a  slow  intake  of  water;  and  (3)  differences  in  the 
osmotic  pull  that  plants  exert  on  the  soil-water.  The  last  two 
factors  argue  for  different  wilting  coefficients  for  crops  on  the 
same  soil. 

The  extended  work  of  Briggs  and  Shantz,2  however,  indi- 
cate that  the  permanent  wilting  point,  expressed  as  a  soil- 
moisture  percentage,  is  practically  the  same  for  all  plants. 
Later  Caldwell 3  demonstrated  that  this  relationship  of  the 
physical  constants  of  the  soil  to  the  wilting  point  depends  on 
the  rate  at  which  the  plant  loses  water,  showing  that  the  soil 
factors  are  not  entirely  dominant  in  this  respect. 

The  conclusions  of  Briggs  and  Shantz,  nevertheless,  seem 

1  Alway,  F.  J.,  Studies  on  the  Relation  of  the  Non-available  Water  of 
the  Soil  to  the  Hygroscopic  Coefficient;  Nebr.  Agr.  Exp.  Sta.,  Res.  Bui.  3, 
1913. 

2  Briggs,  L.  J.,  and  Schantz,  H.  L.  The  Wilting  Coefficient  for  Dif- 
ferent Plants  and  its  Indirect  Determination,  U.  S.  Dept.  Agr.,  Bur. 
Plant  Ind.,  Bui.  230,  1912. 

3  Caldwell,  J.  S.,  The  Relation  of  Environmental  Conditions  to  the 
Phenomena  of  Permanent  Wilting  in  Plants;  Physiological  Researches, 
Station  N,  Baltimore;  U.  S.  Dept.  Agr.,  Vol.  1,  No.  1,  July,  1913. 


196         NATURE  AND  PROPERTIES  OF  SOILS 

more  or  less  accurate  for  plants  growing  under  humid  condi- 
tions. If  such  is  the  case,  it  can  be  accounted  for  only  by  the 
fact  that  the  soil  forces  in  their  effect  on  the  wilting  point 
are  so  powerful  as  to  over-ride  any  distinguishing  character- 
istics that  the  plant  itself  may  possess,  or  at  least  reduce  such 
influences  within  the  error  of  actual  experimentation.  Crops 
wilt  because  they  cannot  get  water  fast  enough,  the  wilting 
coefficient  in  a  humid  climate  being  the  same  for  most  plants 
growing  on  the  same  soil. 

Briggs  and  Shantz,1  in  their  investigations,  devised  a  very 
satisfactory  method  for  making  determinations  of  the  wilting 
point.  Glass  tumblers  holding  about  250  cubic  centimeters 
of  soil  in  an  optimum  condition  were  used.  The  seeds  were 
placed  in  this  soil  after  which  soft  paraffin  was  poured  over 
the  surface  in  order  to  stop  evaporation,  thus  removing  this 
disturbing  factor  in  the  capillary  equilibrium  of  the  moisture. 
The  seedlings  on  germination  were  able  to  push  through  this 
paraffin.  While  the  plants  were  developing,  the  tumblers 
were  kept  standing  in  a  constant-temperature  vat  of  water 
in  order  to  prevent  condensation  of  moisture  on  the  inside 
of  the  glass.  The  vegetative  room  was  under  temperature 
control.  When  definite  wilting  occurred,  as  determined  in 
a  saturated  atmosphere,  a  moisture  determination  was  made 
on  the  soil.  The  resulting  figure,  expressed  as  percentage 
of  moisture  based  on  dry  soil,  represents  the  wilting  coefficient 
for  the  soil  used.2 

It  is  evident  that  the  wilting  coefficient  will  be  influenced 

1  Briggs,  L.  J.,  and  Schantz,  H.  L.,  The  Wilting  Coefficient  for  Differ- 
ent Plants  and  its  Indirect  Determination;  U.  S.  Dept.  Agr.,  Bur.  Plant 
Ind.,  Bui.  230,  pp.  26-33,  1912. 

2Bouyoucos  classifies  the  capillary  water  into  two  groups,  Free  (the 
more  active),  and  Capillary-absorbed  (inner  capillary).  The  distinction 
is  made  on  the  basis  of  his  dilatometer  (see  foot-note,  page  155)  results, 
the  portion  which  freezes  at  about  0°C  being  considered  the  more 
active.  The  point  so  established  by  his  dilatometer  gives  in  a  general 
way  the  wilting  coefficient  as  defined  by  Briggs  and  Shantz. 

Bouyoucos,  G.  J.,  A  New  Classification  of  the  Soil  Moisture;  Soil  Sci., 
Vol.  XI,  No.  1,  pp.  33-47,  Jan.,  1921. 


WATER  OF  SOIL  IN  ITS  RELATION  TO  PLANTS    197 

by  a  number  of  soil  conditions.  Important  among  these  is 
texture,  which  in  itself  really  represents  a  group  of  soil  con- 
ditions. In  general  the  wilting  point  is  much  higher  on  a 
fine  soil  than  one  of  a  coarse  nature.  The  following  data  from 
Briggs  and  Shantz  1  is  interesting  in  this  regard.  The  wilt- 
ing coefficient  is  shown  to  lie  much  nearer  the  hygroscopic 
coefficient  than  to  the  figure  representing  the  maximum  ab- 
sorption capacity  as  determined  by  the  Hilgard  method. 

Table  XXXVII 

RELATION  OF  THE  WILTING  COEFFICIENT  TO  THE  TEXTURE  OF  THE 

SOIL,  THE  HYGROSCOPIC  COEFFICIENT  AND  THE  CALCULATED 

MAXIMUM  ABSORPTIVE  CAPACITY  OF  THE  SOIL 

FOR  WATER. 


Soil 

Hygroscopic 
Coefficient 

Wilting  Point 

Calculated 

Maximum 

Absorption 

Capacity 

Coarse  sand 

Fine  sand 

.5 
1.5 
2.3 
3.5 
4.4 
6.5 
7.8 
9.8 
11.4 

.9 

2.6 

3.3 

4.8 

6.3 

9.7 

10.3 

13.9 

16.3 

25.7 

28  5 

Fine  sand 

30.5 

Sandy  loam 

Sandy  loam 

Fine  sandy  loam  . . 
Loam 

34.9 
39.2 
49(.l 
50.8 

Loam 

61.3 

Clay  loam 

68.2 

In  studying  the  correlation  of  this  wilting  coefficient  to 
soil  conditions  Briggs  and  Shantz  2  advanced  the  following 
relationships.    Expressed  as  formulas,  they  represent  methods 

1  Briggs,  L.  J.,  and  Schantz,  H.  L.,  The  Wilting  Coefficient  for  Dif- 
ferent Plants  and  its  Indirect  Determination;  U.  S.  Dept.  Agr.,  Bur. 
Plant  Ind.,  Bui  230,  p.  65,  1912. 

See  also  Heinrich,  E.,  voer  das  Vermogen  der  Pflanzen  den  Bodenen 
Wasser  zu  erschopfen;  Jahresbericht  der  Agr.-ehem.,  Band  18,  Seite  368- 
372,  1875. 

2  Briggs,  L.  J.,  and  Shantz,  H.  L.,  The  Wilting  Coefficient  for  Dif- 


198         NATURE  AND  PROPERTIES  OF  SOILS 

of  at  least  approximating  the  wilting  point  from  other  soil 
factors.  These  formulas 1  are  arranged  in  the  order  of  their  re- 
liability, based  on  the  data  obtained  by  the  authors : 

„    ™-.,  .  /v.  •  Moisture  equivalent 

1.  Wilting  coefficient  = =-«j 

o    wn-  &  •     *        Hygroscopic  coefficient 

2.  Wilting  coefficient  =  — — £— ^ 

Water-holding  capacity — 21 
( Hil  gar  d  Method) 


3.  Wilting  coefficient 


2.9 


While  such  formulae  are  only  approximate  in  their  applica- 
tion, they  are  valuable  for  rough  calculations.  They  also  show 
in  a  general  way  the  correlations  between  the  various  moisture 
conditions  established  by  experimental  methods. 

105.  The  availability  of  the  soil-water. — From  the  dis- 
cussions already  presented  regarding  the  forms  of  water  in 
the  soil,  the  ways  in  which  they  are  held,  and  their  movements, 
it  is  evident  that  all  moisture  present  in  a  soil  is  not  available 
for  plant  growth.  Three  divisions  of  the  soil-water  may  be 
made  on  this  basis:  unavailable,  available,  and  superfluous. 

It  is  obvious  that  all  of  the  moisture  below  the  wilting  point 
is  out  of  reach  of  the  plant  and  may  be  classified  as  unavail- 
able. It  includes  all  of  the  hygroscopic  and  that  part  of  the 
capillary  which  is  tightly  held,  the  so-called  inner  capillary 
water.  The  amount  of  the  capillary  moisture  unavailable  to 
plants  is  much  greater  with  clayey  than  with  sandy  soils.  For 
example,  a  sand  with  a  hygroscopic  coefficient  of  1.5  per  cent. 

ferent  Plants  and  its  Indirect  Determination;  U.  S.  Dept.  Agr.,  Bur. 
Plant  Ind.,  Bui.  230,  pp.  56-77,  1912. 

See  also,  Loughridge,  K.  H.,  Investigations  in  Soil  Physics;  Calif.  Agr. 
Exp.  Sta.,  Rep.  1892-3-4,  pp.  70-100,  1894.  Alway,  F.  J.,  and  Clarke, 
V.  L.,  Use  of  Two  Indirect  Methods  for  the  Determination  of  the 
Hygroscopic  Coefficient  of  Soils;  Jour.  Agr.  Ees.,  Vol.  VII,  No.  8,  pp. 
345-359,  Nov.,  1916. 

1  Note  that  the  wilting  coefficient,  moisture  equivalent,  water-holding 
capacity  and  hygroscopic  coefficient  are  expressed  in  percentage  of 
water  based  on  dry  soil. 


WATER  OF  SOIL  IN  ITS  RELATION  TO  PLANTS    199 

and  a  wilting  coefficient  of  2.6  per  cent,  has  1.1  per  cent,  of 
water  of  a  capillary  nature  unavailable.  A  clay  loam  having 
a  hygroscopic  coefficient  of  11.4  per  cent,  and  a  wilting  coeffi- 
cient of  16.3  per  cent,  would  contain  4.9  per  cent,  of  capillary 
water  unavailable  to  crops.  It  must  be  remembered,  however, 
that  under  certain  conditions  plants  may  reduce  the  capillary 
moisture  almost  to  the  hygroscopic  coefficient.1  The  moisture 
so  obtained  is  probably  not  utilized  for  growth  activities. 

Advancing  from  the  wilting,  or  critical,  moisture  content  of 
a  soil,  all  the  remaining  capillary  water  is  found  to  be  avail- 

HYGRO.       WILTING      .     LENTO-  MAX.  FIELD 

COEFF.  X    (JPOINT      (CAPILLARITY  CAPACITY 

*\  ^k.        V   OPTIMUM  WATER  ZONE  fl/ 


OPTIMUM  WATER  ZONE 


HYGROS — .]«         CAPILLARY  WATER +GRAVITAT10NAL 

COPIC  WATER1  i  !        WATER 

y >r*j y ^ y 

UNAVA1LABLE\  AVAILABLE  SUPERFLUOUS 

AVAILABJ-E     UNDER 
CERTAIN     CONDITIONS 

Fig.  35. — Diagram  showing  the  various  forms  of  water  that  may  be 
present  in  the  soil  and  their  relations  to  higher  plants. 

able  for  normal  plant  use.  However,  when  free  water  begins 
to  appear,  a  condition  detrimental  to  growth  is  established, 
conditions  becoming  more  adverse  as  the  saturation  point  is 
approached.  This  free  water  is  designated  as  the  superfluous 
water  and  its  presence  generates  conditions  unfavorable  to 
plants.  The  bad  effects  of  free  water  on  the  plant  arise 
largely  from  the  poor  aeration  that  results  from  its  presence.2 
Not  only  are  the  roots  deprived  of  their  oxygen,  but  toxic 
materials  tend  to  accumulate.  Favorable  bacterial  activities, 
such  as  the  production  of  ammonia  and  nitrates,  are  much  re- 

1  Alway,  F.  J.,  Studies  on  the  Relation  of  the  Non-available  Water 
of  the  Soil  to  the  Hygroscopic  Coefficient;  Nebr.  Agr.  Exp.  Sta.,  Res. 
Bui.  3,   1913. 

2  It  must  be  kept  in  mind  that  in  a  clayey  soil  the  superfluous  water 
may  include  some  of  the  upper  capillary  moisture. 


200 


NATURE  AND  PROPERTIES  OF  SOILS 


tarded  also.  The  various  forms  of  water  in  the  soil  and  their 
availability  to  the  plant  are  illustrated  diagrammatically  in 
Fig.  35,  page  199. 

This  diagram  may  be  evaluated  in  a  general  way  as  below, 
using  the  sandy  and  clayey  soils  for  which  full  physical  data 
have  already  been  given  in  Chapter  VIII.  (See  footnote  on 
page  179.) 

Table  XXXVIII 

THE   EVALUATION    OF   FIG.    35    FOR   A    SANDY   AND    CLAYEY   SOIL, 
RESPECTIVELY. 


Hygroscopic  coefficient. 

Wilting   point 

Maximum  field  capacity 

Unavailable  water 

Available  water 

Superfluous  water 


Sandy  Soil 

Clayey  Soil 

1.00 

10.00 

1.47 

14.70 

17.00 

44.00 

1.47 

14.70 

15.53 

29.30 

8.00 

1.60 

106.  Optimum  moisture  for  plant  growth. — It  is  very 
evident  that  there  must  be  some  moisture  condition  of  a  soil 
which  is  best  for  plant  development.  This  is  usually  desig- 
nated as  the  optimum  content.  It  is  not  to  be  assumed,  how- 
ever, that  the  total  range  of  the  available  soil-water  repre- 
sents this  condition.  Nor  is  this  optimum  water  content  in 
any  particular  soil  to  be  designated  by  a  definite  percentage. 
In  reality  the  moisture  in  a  soil  may  undergo  considerable 
fluctuation  and  yet  allow  the  plant  to  develop  normally.1  This 
is  because  the  physical  condition  of  the  soil  changes  with 
varying  water  content  and  the  plant  is  able  to  accommodate 

1Wollny,  E.,  Untersuchung  iioer  den  Einfluss  der  WachsthumsfaJctoren 
auf  des  Produktionsvermbgen  der  Kulturpflanzen  ;  Forsch.  a.  d.  Gebiete 
d.  Agri.-Physik.,  Band  20,  Seite  53-109,  1897. 

Mayer,  A.,  t!oer  den  Einfluss  Tcleinerer  oder  grosser er  Mangen  von 
Wasser  auf  die  Entwickelung  einiger  Kulturpflanzen ;  Jour.  f.  Landw., 
Band  46,  Seite  167-184,  1898. 


WATER  OF  SOIL  IN  ITS  RELATION  TO  PLANTS    201 

itself  to  such  a  fluctuation  without  a  disturbance  in  its  normal 
development.  Granulation  has  considerable  influence  on  the 
range  of  optimum  moisture  conditions,  since  the  better  the 
granulation,  the  better  able  is  the  soil  to  accommodate  itself 
to  changes  in  water  content  without  a  disturbance  of  normal 
growth.  In  moisture  conservation  and  control,  a  granular 
soil  is  one  of  the  first  improvements  to  be  aimed  at.  Drainage, 
liming,  addition  of  organic  matter,  and  tillage,  by  leading  up 
to  such  a  condition,  increase  the  effectiveness  and  economy  of 
soil  moisture  utilization. 

Many  of  the  ordinary  farming  operations  have  to  do  with 
the  maintenance  of  an  optimum  moisture  condition  in  the  soil. 
During  periods  of  excessive  rainfall,  especially  during  the 
growing  season,  conditions  should  be  such  as  to  allow  the  pres- 
ence of  free  water  in  the  soil  for  the  briefest  time  possible. 
This  means  adequate  under-drainage  and  satisfactory  arrange- 
ments whereby  the  run-off  may  be  removed  with  but  little 
damage.  Moisture  control  also  demands  conservation  meth- 
ods of  more  or  less  intensity  in  arid  and  semi-arid  regions  sup- 
plemented by  irrigation,  whereby  the  soil-moisture  may  never 
drop  much  below  the  point  of  lento-capillarity.  By  such  ar- 
rangments  the  optimum  moisture  conditions,  so  essential  to 
normal  and  uninterrupted  crop  growth,  are  maintained. 


CHAPTER  X 
THE  CONTROL  OF  SOIL-MOISTURE 

In  the  discussion  of  the  water  requirements  of  plants  it 
was  apparent  that  for  a  normal  yield  of  any  crop,  the  amount 
used  by  the  plant  alone  was  very  great,  varying  from  five  to 
ten  acre-inches  according  to  conditions.  Were  this  the  only 
loss  of  water,  the  question  of  raising  crops  with  given  amounts 
of  rainfall  would  be  a  simple  one.  Three  further  sources  of 
water  loss,  however,  are  usually  operating  in  the  soil  and  tend- 
ing to  lower  the  water  that  would  go  toward  transpiration,  a 
loss  absolutely  necessary  for  proper  growth.  The  various 
ways  by  which  water  finds  an  exit  from  a  soil  are:  (1)  tran- 
spiration, (2)  run-off  over  the  surface,  (3)  percolation,  and 
(4)  evaporation.  The  diagram  (Fig.  36)  makes  clear  their 
relationships. 

It  is  immediately  obvious  that,  as  the  losses  by  run-off, 
leaching,  and  evaporation  increase,  the  amount  of  water  left 
for  crop  utilization  decreases.  Some  control  of  soil-water 
is,  therefore,  necessary  both  in  an  arid  and  a  humid  region. 
Under  arid  and  semi-arid  conditions,  where  run-off  and  per- 
colation are  not  of  such  great  importance  except  where  irriga- 
tion is  practiced,  loss  by  evaporation  is  of  especial  consequence, 
as  it  competes  directly  with  the  plant.  Under  humid  condi- 
tions, losses  by  percolation  and  run-off  seem  to  merit  the 
greater  attention,  because  of  the  loss  of  nutrients  with  the 
former  and  the  erosion  damage  from  the  latter.  The  influence 
of  evaporation,  however,  is  not  to  be  under-estimated  or  ne- 
glected. Control  of  moisture  is,  therefore,  necessary  in  all 
regions.     This  control  consists  in  so  adjusting  run-off,  leach- 

202 


THE  CONTROL  OF  SOIL-MOISTURE 


203 


ing,  and  evaporation  as  to  maintain  optimum  moisture  condi- 
tions in  the  soil  at  all  times.  Such  control  should  result  in 
a  proper  and  economical  utilization  of  soil-water  by  the  plant. 
107.  Run-off  losses. — In  regions  of  heavy  rainfall  or 
in  areas  where  the  land  is  sloping  or  rather  impervious  to 
water,  a  considerable  amount  of  moisture  received  as  rain  is 
likely  to  be  lost  by  running  away  over  the  surface.    Under 


TRAN  SP/KAT/ON. 


Fig.  36. — Diagram  illustrating  the  various  ways  by  which  water  may  be 
lost  from  a  soil. 


such  conditions  two  considerations  are  important:  (1)  the  loss 
of  water  that  might  otherwise  be  of  use  to  plants;  and  (2)  the 
erosion  that  usually  occurs  when  much  water  escapes  in  this 
manner.  Of  the  two,  the  latter  is  generally  the  more  impor- 
tant. The  amount  of  run-off  varies  with  the  rainfall  and  its 
distribution,  the  slope,  the  character  of  the  soil,  and  the  vege- 
tative covering.  In  some  regions  loss  by  run-off  may  rise  as 
high  as  50  per  cent,  of  the  rainfall,  while  in  arid  sections  it 
is  of  course  very  low,  unless  the  rainfall  is  of  the  torrential 
type  as  in  the  arid  Southwest. 


204         NATURE  AND  PROPERTIES  OF  SOILS 

The  quantity  of  water  entering  a  soil  is  determined  almost 
entirely  by  the  physical  condition  of  the  soil.  If  it  is  loose 
and  open,  the  water  enters  readily  and  little  is  lost  over  the 
surface  as  run-off.  If,  on  the  other  hand,  the  soil  is  com- 
pact, impervious  and  hard,  most  of  the  rainfall  runs  away, 
and  not  only  is  there  a  serious  loss  of  water,  but  considerable 
erosion  may  also  result.  The  first  step  in  checking  run-off 
losses,  therefore,  is  strictly  physical  in  nature.  Good  tillage 
and  plenty  of  organic  matter  by  encouraging  granulation  have 
much  to  do  with  the  proper  entrance  of  water  into  the  soil  as 
well  as  with  its  economic  utilization  therein. 

108.  Erosion  by  water  and  its  control.1 — While  every 
one  is  familiar  with  the  importance  of  water  in  the  forma- 
tion of  alluvial  and  marine  soils,  the  concurrent  destructive 
action  that  is  going  on  in  the  uplands  is  generally  overlooked. 
This  is  due  to  the  fact  that  erosion  is  often  considered  as  more 
or  less  uncontrollable,  an  ill  that  can  not  be  avoided.  In 
Wisconsin,  for  example,  50  per  cent,  of  the  tillable  land  is 
subject  to  erosion  of  economic  importance.2  Even  in  as  level  a 
state  as  Illinois,  17  per  cent,  of  the  area  is  detrimentally 
eroded.3  The  waste  by  erosion  is  as  great  in  other  states, 
even  those  of  an  arid  climate.  Davis  4  has  estimated  that  870 
million  tons  of  suspended  material  are  carried  each  year  into 
the  ocean  by  the  streams  of  the  United  States.  Since  this  is 
only  a  very  small  fraction  of  the  soil  brought  down  from  the 

1  Davis,  E.  O.  E.,  Economic  Waste  from  Soil  Erosion;  U.  S.  Dept. 
Agr.,  Year  Book  for  1913,  pp.  207-220. 

Ramser,  C.  E.,  Terracing  Farm  Lands;  U.  S.  Dept.  Agr.,  Farmers'  Bui. 
997,  1918. 

Eastman,  E.  E.,  and  Glass,  J.  S.,  Soil  Erosion  in  Iowa;  la.  Agr.  Exp. 
Sta.,  Bui.  183,  Jan.,  1919. 

Fisher,  M.  L.,  The  Washed  Lands  of  Indiana;  Ind.  Agr.  Exp.  Sta., 
Circ.  90,  Feb.,   1919. 

3Whitson,  A.  R.,  and  Dunnewald,  T.  J.,  Keep  Our  Hillsides  from 
Washing;  Wis.  Agr.  Exp.  Sta.,  Bui.  272,  Aug.,  1916. 

3  Mosier,  J.  G.,  and  Gustaf son,  A.  F.,  Soil  Physics  and  Management, 
p.  358,  Philadelphia,  1917. 

4  Davis,  R.  O.  E.,  Economic  Waste  from  Soil  Erosion;  U.  S.  Dept.  Agr., 
Year  Book  for  1913,  p.  213. 


THE  CONTROL  OF  SOIL-MOISTURE  205 

uplands  by  running  water,  erosion  is  no  insignificant  factor 
in  soil  management  considerations. 

Two  types  of  erosion  are  generally  recognized,  sheet  and 
gully.  In  the  former,  soil  is  removed  more  or  less  uniformly 
from  every  part  of  the  slope.  Gullying  occurs  where  the  vol- 
ume of  water  is  concentrated,  resulting  in  the  formation  of 
ravines  by  undermining  and  downward  cutting.  Both  types 
of  erosion  are  serious. 

A  number  of  different  methods  for  the  effective  prevention 
and  control  of  erosion  may  be  utilized.  Anything  that  will 
increase  the  absorptive  capacity  of  the  soil,  such  as  deep  plow- 
ing, surface  tillage,  and  increase  of  organic  matter,  will  lessen 
the  run-off  over  the  surface.  On  steep  slopes,  however,  such 
influence  is  of  little  importance,  since  during  heavy  rainfall 
absorption  is  too  slow  to  lessen  materially  the  surface  losses. 
In  cultivating  corn  and  similar  crops,  it  is  important  that  the 
last  cultivation  be  across  the  slope  rather  than  with  it.  On 
long  slopes  subject  to  erosion,  the  fields  may  be  laid  out  in  long 
narrow  strips  across  the  incline,  alternating  the  tilled  crops, 
such  as  corn  and  potatoes,  with  hay  and  grain.  The  grassed 
areas  tend  to  check  the  surface  flow  of  water.  Where  the 
slopes  are  subject  to  very  serious  erosion,  they  should  either  be 
reforested  or  kept  in  permanent  pasture,  guarding  always 
against  incipient  gullying. 

About  the  only  effective  means  of  controlling  sheet  erosion 
is  by  terracing  of  some  kind.  Strong  prejudice  exists  in  many 
communities  against  terraces,  since  they  usually  waste  land, 
are  often  unsightly  and  are  a  serious  obstacle  to  harvesting 
machinery.  The  Mangum  terrace  *  however,  is  worthy  of  es- 
pecial attention,  since  it  obviates  the  really  serious  objections 
to  the  ordinary  terrace  while  maintaining  the  desired  water 
control.  The  Mangum  terrace  is  generally  a  broad  bank 
of  earth  with  gently  sloping  sides,  contouring  the  field  at  a 

1  First  constructed  by  P.  H.  Mangum  of  Wake  County,  North  Caro- 
lina. 


206         NATURE  AND  PROPERTIES  OF  SOILS 

grade  from  10  to  12  inches  to  the  100  feet.  It  is  usually 
formed  by  back-furrowing  and  scraping.  The  interval  be- 
tween the  embankment  depends  on  the  slope.  Since  the  terrace 
is  low  and  broad,  it  may  be  cropped  without  difficulty  and 
offers  no  obstacle  to  cultivating  and  harvesting  machinery. 
It  wastes  no  land,  and  eliminates  breeding  places  for  insects. 

Small  gullies,  while  at  first  insignificant,  soon  enlarge  into 
deep  unsightly  ravines.  While  they  may  be  plowed-in  or 
otherwise  filled  up,  such  a  procedure  is  generally  a  waste  of 
time,  since  the  gullies  form  again  with  the  next  heavy  wash. 
A  number  of  different  methods  are  in  use  for  the  control  of 
gullying,  depending  on  conditions.  Staking  is  a  very  common 
procedure,  the  size  of  the  stakes  increasing  with  the  magni- 
tude of  the  gully.  The  stakes  are  usually  interwoven  with 
brush,  although  stone,  straw,  and  other  material  may  be 
utilized.  If  brush  or  other  loose  material  is  used,  it  should 
be  staked  to  the  ground  or  held  down  by  stone  or  dirt.  Other- 
wise, the  water  will  run  beneath  the  fill  and  no  benefit  will 
result.  Dams  of  earth,  concrete,  or  stone  are  often  installed 
with  success.  They  must  be  supplemented  by  a  tile-drain 
outlet,  however,  with  an  elbow  just  above  the  dam.  The  dam 
checks  the  water  until  it  rises  to  the  level  of  the  elbow  outlet 
and  is  then  carried  away  through  the  tile.  Most  of  the  sedi- 
ment is  deposited  above  the  dam  and  the  gully  is  slowly  filled. 

109.  Percolation  losses  and  their  control. — When  at 
any  time  the  amount  of  rainfall  entering  a  soil  becomes  greater 
than  its  water-holding  capacity,  losses  by  percolation  will 
result.  The  losses  will  depend  largely  on  the  amount  and 
distribution  of  the  rainfall  and  the  capability  of  the  soil  to 
hold  moisture.  The  objectionable  features  of  excessive  per- 
colation are  two:  (1)  the  actual  loss  of  water,  and  (2)  the 
leaching-out  of  salts  that  may  function  as  nutrients  to  plants. 

The  results  from  the  Rothamsted  lysimeter x  from  1871- 

1  Hall,  A.  D.,  The  Booh  of  the  Bothamsted  Experiments,  p.  22,  New 
York,  1917. 


THE  CONTROL  OF  SOIL-MOISTURE 


207 


1913  on  a  bare  clay  loam  three  feet  deep  are  interesting  as  to 
the  light  they  afford  regarding  actual  drainage  losses  in  humid 
regions : 

Table  XXXIX 

PERCOLATION    THROUGH    A    SLXTY-INCH    COLUMN    OF    BARE    CLAY 

LOAM.      ROTHAMSTED  EXPERIMENT  STATION,  ANNUAL 

AVERAGE   OF    42    YEARS. 


Periods 

Eainfall 
Inches 

Drainage 
Inches 

Percentage 

OF  EAINFAIaL 

as  Drainage 

Dec.-Feb 

Mar.-May 

June-Aug 

6.77 
5.96 
7.83 
8.29 

5.58 
2.11 
1.82 
4.50 

82.4 
35.4 
23.2 

Sept.-Nov 

54.2 

Mean  Total 

28.85 

14.01 

48.8 

It  appears  from  these  figures  that  the  drainage  loss  is  much 
lower  in  summer  than  winter,  the  ratio  being  about  one  to 
three.  It  is  also  to  be  noted  that  about  50  per  cent,  of  the 
rainfall  in  such  a  climate  as  England  is  lost  by  percolation 
through  a  bare  soil.  This  compares  fairly  well  with  Wollny's x 
summary  on  eighteen  soils  in  England,  Switzerland,  and  Ger- 
many. These  soils,  most  of  which  were  bare,  showed  a  loss 
of  over  41  per  cent,  of  the  rainfall  by  drainage. 

Recent  results,2  due  to  variable  conditions,  are  by  no  means 
in  agreement,  ranging  from  a  low  to  a  very  high  percentage 
loss  of  the  rainfall.  It  seems  fair  to  assume,  however,  that, 
as  soils  are  handled  in  humid  regions,  over  half  of  the  rain- 
fall is  lost  by  percolation  and  run-off  combined. 

Percolation  seems  to  be  influenced,  not  only  by  the  amount 

1Wollny,  E.,  Untersuchungen  uber  die  Siclcerwassermengen  in  verscMe- 
denen  Bodenarten;  Forsch.  a.  d.  Gebiete  d.  Agri.-Physik.,  Band  11, 
Seite  1-68,  1888. 

aFor  excellent  review  of  literature  see  Lyon,  T.  L.,  and  Bizzell, 
J.  A.,  Lysimeter  Experiments ;  Cornell  Agr.  Exp.  Sta.,  Memoir  12,  June, 
1918. 


208         NATURE  AND  PROPERTIES  OF  SOILS 

of  rainfall  and  its  distribution,  but  also  by  evaporation,  the 
character  of  the  soil,  and  the  presence  of  a  crop.  As  the  rain- 
fall increases,  percolation  increases,  being  much  greater  in 
New  York,  for  example,  than  in  Utah.  Evaporation  has  a 
marked  influence,  reducing  drainage  losses  to  a  considerable 
degree.  The  drainage  through  sandy  soils  is  generally  larger 
than  through  clayey  soils  under  strictly  humid  conditions  and 
where  run-off  is  a  factor.  When  evaporation  is  high,  sandy 
soils  have  been  known  to  percolate  very  much  less  than  those 
of  a  heavier  nature.1  Field  crops,  in  that  they  utilize  a  large 
amount  of  moisture,  have  always  been  found  to  reduce  per- 
colation losses. 

The  loss  of  moisture  by  percolation  is  the  least  objectionable 
feature  of  the  phenomenon,  since  it  is  often  necessary,  espe- 
cially during  the  spring  and  summer,  to  rid  the  soil  very 
quickly  of  superfluous  water.  The  loss  of  nutrient  salts  is 
more  vital,  since  the  materials  so  carried  away  might  be  used 
by  plants.  The  loss  of  nitrogen,  calcium,  and  potassium  from 
a  bare  clay  loam  at  Cornell  University  *  over  a  period  of  ten 
years  averaged,  respectively,  69,  398,  and  72  pounds  an  acre 
annually.  This  is  equivalent  to  an  acre  loss  of  419  pounds 
of  sodium  nitrate,  995  pounds  of  calcium  carbonate  and  137 
pounds  of  potassium  chloride  every  year,  which  is  a  larger 
amount  of  nutrient  material  than  is  removed  by  an  average 
crop. 

Control  of  percolation  is  exerted,  not  so  much  to  save  water, 
as  to  conserve  nutrients.  As  water  enters  a  soil  it  moves 
downward  and  is  continually  changing  into  the  capillary  state. 
If  the  absorptive  capacity  of  the  soil  is  high,  little  of  the  rain- 
fall may  appear  as  drainage.  The  presence  of  organic  matter 
and  the  influence  of  good  tillage  will  do  much  toward  check- 
ing drainage  losses.    Once  the  absorptive  capacity  of  the  soil 

1  Fraps,  G.  S.,  Losses  of  Moisture  and  Plant  Food  by  Percolation;  Tex. 
Agr.  Exp.  Sta.,  Bui.  171,  1914. 
"Unpublished  data.    Cornell  Agr.  Exp.  Sta.,  Ithaca,  N.  Y. 


THE  CONTROL  OF  SOIL-MOISTURE 


209 


is  reached,  however,  the  drainage  should  be  as  rapid  and  com- 
plete as  possible  in  order  to  insure  good  sanitation.  The  main- 
tenance of  a  high  absorptive  capacity  for  available  water  and 
the  facilitation  of  rapid  drainage  are  the  secrets  of  rational 
percolation  control. 


Fig.  37. — Influence  of  drainage  on  the  ground  water  and  the  extent  of 
the  root  zone. 


In  this  connection  it  is  well  to  remember  that  drainage  losses 
are  profoundly  affected  by  cropping.  The  following  data  from 
the  Cornell  Experiment  Station  are  especially  interesting  in 
this  regard.  The  data  for  the  Dunkirk  and  Volusia  soils  are 
for  ten  and  fifteen  years  respectively: 

Table  XL 

AVERAGE   ANNUAL   LOSS   OF    WATER   BY   PERCOLATION   FROM   BARE 
AND  CROPPED   SOILS.      CORNELL  LYSIMETER  TANKS. 


Conditions 

Eainfall 
Inches 

Percolation 
Inches 

Eainfall  as 

Percentage  of 

Drainage 

Dunkirk  clay  loam : 
Bare 

32.41 
32.41 

32.97 
32.97 

24.92 
18.70 

27.13 
20.62 

76.8 

Cropped 

57.7 

Volusia  silt  loam: 
Bare 

82.3 

Cropped 

62.5 

210         NATURE  AND  PROPERTIES  OF  SOILS 


Table  XLI 

AVERAGE   ANNUAL    LOSS    OF    NUTRIENTS   BY    PERCOLATION    FROM 
BARE  AND  CROPPED  SOILS.      CORNELL  LYSIMETER  TANKS. 


Conditions 

Annual  Loss  in  Pounds  an  Acre 

NITROGEN 

CALCIUM 

POTASSIUM 

Dunkirk  clay  loam : 
Bare 

69.0 
7.3 
2.5 

51.8 
10.2 

397.9 
247.1 
259.9 

341.4 
356.4 

72.0 

Rotation 

Grass 

57.3 
61.7 

Volusia  silt  loam: 
Bare 

84.5 

Cropped 

73.2 

The  influence  of  the  crop  on  percolation  is  obvious,  the  loss 
of  water  by  drainage  being  markedly  decreased.  The 
saving  of  nutrient  is  also  very  marked,  especially  as  regards 
the  nitrogen.  The  loss  of  nitrogen  is  only  about  one-seventh 
as  much  from  the  soils  under  a  rotation,  as  where  the  land 
was  bare,  while  the  saving  of  calcium  and  potassium  is  con- 
siderable. The  importance  of  catch-  and  cover-crops  in  eco- 
nomical soil  management  need  not  be  emphasized  further. 

110.  Drainage.1 — While  percolation,  especially  in  hu- 
mid regions,  causes  the  loss  of  a  large  proportion  of  the 
rainfall  received  and  carries  away  in  addition  many  tons  of 

1Klippart,  J.  H.,  Principles  and  Practice  of  Land  Drainage;  Cin- 
cinnati, 1894. 

Miles,  M.,  Land  Drainage;  New  York,  1897. 

Faure,  L.,  Drainage  et  Assainissement  Agricole  des  Terres;  Paris, 
1903. 

Elliott,  C.  G.,  Drainage  of  Farm  Lands;  U.  S.  Dept.  Agr.,  Farmers' 
Bui.  187,  1904. 

King,  F.  H.,  Irrigation  and  Drainage,  Eevised  Edition;  Part  II,  New 
York,  1909.  ' 

Warren,  G.  M.,  Tidal  Marshes  and  their  Reclamation;  U.  S.  Dept. 
Agr.,  Office  Exp.  Sta.,  Bui.  240,  1911. 

Woodward,  S.  M.,  Land  Drainage  oy  Means  of  Pumps;  U.  S.  Dept. 
Agr.,  Office  Exp.  Sta.,  Bui.  243,  1911. 

Elliott,  C.  G.,  Engineering  for  Land  Drainage;  New  York,   1912. 

Parsons,  J.  L.,  Land  Drainage;  Chicago,  1915. 


THE  CONTROL  OF  SOIL-MOISTURE  211 

soluble  material,  it  is  generally  wise  to  facilitate  the  rapidity 
of  its  action  while  checking,  if  possible,  its  magnitude.  The 
encouragement  of  the  rate  of  percolate  is  spoken  of  as  land 
drainage,  which  is  the  process  of  removing  the  excess  or 
superfluous  water  from  the  sdil  as  rapidly  as  possible. 
Excess  water,  by  interfering  with  aeration,  sets  up  unsanitary 
conditions  within  the  soil.  By  draining  the  land  many  favor- 
able reactions  are  promoted.  Granulation  is  encouraged, 
heaving  is  checked,  while  the  root  zone  and  water  capacity 
of  the  soil  are  markedly  increased.  By  facilitating  aeration, 
favorable  chemical  and  biological  changes  are  encouraged, 
thus  increasing  the  nutrients  available  for  plants.  The  sum- 
total  of  good  drainage  is  an  increase  of  crop  production  to 
such  an  extent  as  to  meet  the  investment  costs  and  pay  a  hand- 
some profit  besides. 

While  the  drainage  of  swamps  and  the  reclamation  of  over- 
flow areas  are  urgent,  the  drainage  of  lands  already  under 
crop  is  more  important.  Practical  farm  drainage  is  para- 
mount in  almost  every  community,  even  in  arid  regions  where 
irrigation  must  be  practiced.  Two  types  of  drainage  are 
feasible — open  and  closed.  Ditch  drainage  is  the  usual  type 
of  the  first  group.  Ditches  have  the  advantage  of  large  ca- 
pacity and  are  able  to  carry  water  at  a  low  grade.  On  the 
other  hand,  they  waste  land,  are  ineffective  and  inconvenient, 
encourage  erosion  and  demand  a  yearly  up-keep  expenditure. 
Wherever  possible  under-drains  should  be  used. 

Jeffery,  J.  A.,  Text-Boole  of  Land  Drainage;  New  York,  1916. 

Fippin,  E.  O.,  Drainage  in  New  York;  Cornell  Agr.  Exp.  Sta.,  Bui. 
254,  1908. 

Brown,  C.  F.,  Farm  Drainage;  Utah  Agr.  Exp.  Sta.,  Bui.  123, 
1913. 

Lynde,  H.  M.,  Farm  Drainage  in  North  Carolina;  N.  C.  Agr.  Exp. 
Sta.,  Bui.  234,  1915. 

Yarnell,  D.  L.,  Trenching  Machinery  Used  for  the  Construction  of 
Trenches  for  Tile  Drains;  U.  S.  Dept.  Agr.,  Farmers'  Bui.  698,  1915. 

Leidigh,  A.  H.,  and  Gee,  E.  C,  Tile  Drainage;  Tex.  Agr.  Exp.  Sta., 
Bui.  188,  1916. 

Hart,  R.  A.,  The  Drainage  of  Irrigated  Farms;  U.  S.  Dept.  Agr., 
Farmers'  Bui.  805,  1917. 


212         NATURE  AND  PROPERTIES  OF  SOILS 

111.  Tile  drains  are  the  only  reliable  means  of  under- 
drainage  under  all  conditions.  While  stone  drains x  are 
of  value  in  certain  cases,  they  must  always  be  short  and 
are  likely  to  clog.  Besides,  their  drainage  is  slow  and  in- 
efficient. On  silty  soil  they  do  not  long  remain  in  service. 
The  operation  of  the  tile  drain  is  simple.  The  tile,  generally 
about  twelve  inches  long  with  a  diameter  varying  with  the 
water  to  be  carried,  are  laid  end  to  end  in  strings,  on  the 
bottom  of  a  trench  of  sufficient  slope,  a  carefully  protected 
outlet  being  provided.  The  tile  are  then  covered  with  earth, 
straw  or  surface  soil  often  being  placed  directly  around  the 
tile  to  facilitate  the  entrance  of  the  water.  The  superfluous 
water  enters  the  tile  through  the  joints,  mostly  from  the 
sides.  As  a  consequence,  the  tops  of  the  joints  may  be  cov- 
ered with  paper,  cloth  or  even  cemented  in  order  to  prevent 
the  entrance  of  silt  or  quick-sand.  The  function  of  a  tile 
drain  system  is  twofold:  (1)  to  collect  the  superfluous  water 
and  (2)  to  discharge  it  quickly  from  the  land. 

Where  the  land  possesses  considerable  natural  drainage,  the 
tile  are  laid  along  the  depressions.  This  is  spoken  of  as  the 
natural  system  of  drainage  in  that  the  tile  facilitate  the  quick 
removal  of  the  water  from  the  places  of  natural  accumulation. 
Where  the  land  is  level  or  gently  rolling,  it  often  needs  uni- 
form drainage.  A  regular  system  must  then  be  installed. 
This  may  be  either  of  the  fishbone  or  gridiron  style,  or  a 
modification  or  combination  of  the  two,  natural  drainage  being 
taken  advantage  of  where  possible.  Where  springs  or  seep- 
age spots  occur,  cut-off  systems  must  be  devised.  (See  Figs. 
38  and  39.) 

1  Stone  drains  are  built  by  arranging  stone  in  a  properly  located  and 
graded  trench  in  such  a  manner  as  to  provide  a  continuous  channel 
or  throat  from  the  upper  end  of  the  drain  to  the  lower.  One  of  the 
safest  modes  of  construction  from  the  standpoint  of  clogging  is  to  place 
flat  stone  on  edge  in  the  trench  with  their  faces  parallel  to  the  walls 
of  the  ditch.  The  spaces  between  the  stone  provide  for  the  movement 
of  the  drainage  water. 


THE  CONTROL  OF  SOIL-MOISTURE 


213 


Every  regular  system  consists  of  two  parts,  the  laterals  and 
the  main  drain.  The  laterals  are  usually  constructed  of 
three  3-  or  4-inch  tile,  seldom  smaller.  These  laterals  should 
always  enter  the  main  at  an  angle  of  about  45  degrees.  This 
causes  a  joining  of  the  currents  with  no  loss  of  impetus  and 


i 
Ground  ^n         /    /  J 


4W  / 


J 


W/ 


/ 


/ 


{ 


\\ 


X 


>\ 


\ 


\\nJ) 
V     / 


\ 


\N 


L 


s 


k 


Fig.   38. — Natural    (1)    and   interception    (2) 
drains. 


for   laying   tile 


allows  the  more  rapidly  moving  lateral  streams  to  speed  up 
the  flow  in  the  main  drain.  The  size  of  the  main  depends  on 
the  rainfall,  the  area  drained,  and  the  slope.  It,  of  course, 
must  be  larger  near  the  outlet  than  at  any  other  point.  The 
following  practical  table  from  Elliott x  indicates  the  influence 

1  Elliott,  C.  GL,  Engineering  for  Land  Drainage;  p.  108,  New  York, 
1912. 


214 


NATURE  AND  PROPERTIES  OF  SOILS 


of  area  and  slope  on  the  size  of  the  main  near  the  outlet  of 
any  system: 

Table  XLI 

GRADES  TO  A  HUNDRED  FEET  IN  DECIMALS  OP  A  FOOT  WITH  AP- 
PROXIMATE EQUIVALENTS  IN  INCHES. 


Grades  to  a  Hundred  Feet 

in  Decimals  of  a  Foot  with 

Diameter 

Approximate  Equivalents  in  Inches 

of  Tile 

(in  Inches) 

Vi  inch 

linch 

2  inches 

3  inches 

6  inches 

9  inches 

0.04 

0.08 

0.16 

0.25 

0.50 

0.75 

Acres 

Acres 

Acres 

Acres 

Acres 

Acres 

5 

17.3 

19.1 

22.1 

25.1 

32.0 

37.7 

6 

27.3 

29.9 

34.8 

39.6 

50.5 

59.4 

7 

39.9 

44.1 

51.1 

58.0 

74.5 

87.1 

8 

55.7 

61.4 

71.2 

80.9 

103.3 

121.4 

9 

74.7 

82.2 

95.3 

108.4 

138.1 

162.6 

10 

96.9 

106.7 

123.9 

140.6 

179.2 

211.1 

12 

152.2 

167.7 

194.6 

221.1 

281.8 

331.8 

The  grade  necessary  for  the  satisfactory  operation  of  a  tile 
drain  system  varies  with  the  system  itself  and  the  portion 
under  consideration.  The  grade  of  the  main  drain  may  be 
very  low,  especially  if  the  laterals  deliver  their  water  with  a 
high  velocity.  In  general,  the  grade  will  vary  from  4  to  20 
inches  to  the  hundred  feet,  8  inches  being  more  or  less  ideal. 
The  depth  of  the  tiles  beneath  the  surface  and  the  distance 
between  laterals  will  vary  with  the  soil.  With  sandy  soils 
the  tile  may  be  placed  as  deep  as  3  or  4  feet.  With  clayey 
soils  the  depth  must  be  shallower,  ranging  from  15  to  30 
inches,  while  the  interval  is  reduced  as  the  soil  becomes  finer 
in  texture.  On  a  clayey  soil  the  distance  between  the  strings 
is  sometimes  as  low  as  35  feet  although  50  to  70  feet  is  com- 
moner. 

The  maintenance  cost  of  a  tile  drain  system  is  low,  the  only 
especial  attention  needful  being  at  the  outlet.     The  outlet 


THE  CONTROL  OF  SOIL-MOISTURE 


215 


should  be  well  protected,  so  that  the  end  tiles  may  not  be 
loosened  and  the  whole  system  endangered  by  clogging  with 
sediment.  It  is  well  to  embed  the  end  tile  in  a  masonry  or 
concrete  wall.  The  last  eight  or  ten  feet  of  tile  may  even  be 
replaced  by  a  galvanized  iron  pipe  or  with  sewer  tile,  thus 


^j  i 


H, 


Fig.  39. — Gridiron  and  fishbone  systems  for  laying  tile  drains. 


insuring  against  damage  by  frost.  The  water  should  flow 
freely  from  a  tile  drain  system,  as  a  drowned  outlet  inter- 
feres with  efficient  drainage.  The  opening  of  a  tile  drain  sys- 
tem is  usually  protected  by  a  gate  or  by  wire  in  such  a  man- 
ner as  to  allow  the  water  to  flow  out  freely  but  preventing 
rodents  from  entering  in  dry  weather.     (See  Fig.  40.) 

As  with  any  other  improvement,  tile  drainage  must  be  made 


216        NATURE  AND  PROPERTIES  OF  SOILS 

to  pay.  If  rapid  efficient  drainage  can  not  be  assured  at  a 
reasonable  cost  and  under  such  conditions  that  the  increased 
crops  will  return  a  good  profit  on  the  investment,  tile  drains 
should  not  be  installed. 

112.  Evaporation  losses. — Evaporation  of  soil-water  takes 
place  almost  entirely  at  the  surface,  exceptions  being 
where  large  cracks  occur,  which  allow  thermal  loss  directly 
from  the  subsoil.  This  loss  of  water  by  direct  evaporation 
from  the  soil  may  be  excessive  and  may  result  in  direct  reduc- 


Fig.  40. — Cement  block  at  the  outlet  of  a  tile  drain. 

tion  of  crop  yield,  a  type  of  loss  so  familiar  that  examples 
hardly  need  be  cited.  In  the  results  with  the  Rothamsted  rain 
gauges  (see  page  207),  about  50  per  cent,  of  the  annual  rain- 
fall was  regained  in  the  drainage  water.  Since  the  gauges  bore 
no  crop,  the  remaining  50  per  cent,  must  have  been  lost  by 
evaporation.  It  should  be  noted  that  in  the  summer  months 
under  warm  temperature,  this  loss  was  greatest,  amounting 
to  75  per  cent,  of  the  rainfall.  Correspondingly,  in  the  semi- 
arid  and  arid  sections  of  the  country  where  there  is  little  or 
no  drainage,  the  rainfall  is  almost  all  lost  by  evaporation. 
Evaporation  from  land  surface  has  an  appreciable  effect  on 


THE  CONTROL  OF  SOIL-MOISTURE 


217 


the  amount  of  rainfall.  Even  in  humid  regions,  where  the 
annual  rainfall  is  ample  for  maximum  crop  production,  the 
yields  are  frequently  reduced  below  the  profit  point  by  pro- 
longed periods  of  dry  weather  in  the  growing  season  during 
which  the  loss  of  water  from  the  plants,  coupled  with  the  loss 
from  the  soil  and  through  weeds,  exhausts  the  moisture  sup- 
ply very  rapidly. 

While  run-off  and  percolation  are  directly  proportional  to 
the  rainfall,  loss  by  evaporation  does  not  vary  to  such  a  de- 
gree. The  loss  by  percolation  depends  almost  directly  on  the 
amount  of  rainfall  above  the  retentive  power  of  the  soil.  In 
years  of  heavy  precipitation  losses  by  percolation  increase. 
Evaporation  from  the  soil  depends  largely  on  the  length  of 
time  that  the  soil  surface  is  moist,  and  this  will  not  vary 
markedly  from  year  to  year.  The  following  figures  from  the 
Rothamsted  x  sixty-inch  drain  gauge  may  be  quoted  in  this 
regard : 

Table  XLIII 

RAINFALL,    DRAINAGE    AND    EVAPORATION    AT    THE    ROTHAMSTED 
EXPERIMENT  STATION,  1871  TO   1912. 


Conditions 

Kainfall 
Inches 

Percolation 
Inches 

Evaporation 
Inches 

Maximum  rainfall,  1903 
Mean  total  for  42  years.  . .  . 
Minimum  rainfall,  1898 

38.69 
28.75 
20.49 

24.23 

13.93 

7.69 

14.46 
15.32 
12.80 

A  rough  calculation  may  be  made  which  will  show  the  ap- 
portionment of  the  yearly  rainfall  in  a  humid  region  of  the 
temperate  zone  between  the  four  forms  of  losses — run-off  and 
percolation,  evaporation,  and  transpiration.  The  percolation 
under  a  rainfall,  say,  of  28  inches,  as  shown  by  the  Rotham- 


1Hall,  A.  D.,  The  Booh  of  the  Bothamsted  Experiments;  p.  22,  New 
York,  1917. 


218        NATURE  AND  PROPERTIES  OF  SOILS 

sted  work,  is  roughly  14  inches.  Run-off  and  percolation  may 
be  considered  as  about  50  per  cent.  The  water  requirement  of 
an  ordinary  crop  is  about  7  inches.  This  leaves  a  loss  of  7 
inches  to  be  credited  to  evaporation.  In  other  words,  in  a 
clay  loam  soil  in  a  climate  like  that  of  England,  one-half 
the  rainfall  goes  as  run-off  and  percolation,  while  the  other 
half  is  divided  about  equally  between  the  plant  and  loss  by 
evaporation.  While  run-off  and  percolation  may  be  checked 
to  some  extent,  not  enough  conservation  can  occur  in  this  di- 
rection to  tide  a  crop  over  a  period  of  drought.  Some  con- 
sideration should,  therefore,  be  directed  towards  the  check- 
ing of  loss  by  evaporation,  since  moisture  thus  saved  means 
just  that  amount  added  to  the  water  available  for  the  use  of 
the  crop  growing  on  the  soil. 

113.  Evaporation  control. — Any  material  applied  to  the 
surface  of  a  soil  primarily  to  prevent  loss  by  evaporation  or  to 
keep  down  weeds  may  be  designated  as  a  mulch.  Mulches  are 
of  two  general  sorts,  artificial  and  natural.  In  the  former  case, 
foreign  material  is  merely  spread  over  the  soil  surface.  Man- 
ure, straw,  leaves,  and  the  like  may  be  used  successfully. 
Such  mulches  while  effective,  especially  in  preventing  weed 
growth,  are  not  generally  applicable  to  field  crops  where  in- 
ter-tillage is  practiced,  since  they  would  make  cultivation  im- 
possible. Their  use  is,  therefore,  limited  to  such  crops  as 
strawberries,  blackberries,  and  the  like. 

The  second  type  of  mulch  is  called  a  soil-mulch  since  it  is 
formed  from  soil  itself.  With  proper  tillage,  a  loose  dry  layer 
of  soil  may  be  formed  on  the  surface.  Such  a  layer  is  designed 
to  obstruct  capillary  movement  to  such  an  extent  as  to  reduce 
evaporation  loss  to  a  minimum.  In  theory  a  soil-mulch  should 
be  formed  as  quickly  as  possible  so  that  the  only  moisture 
sacrificed  will  be  that  which  is  present  in  the  soil  forming 
the  mulch.  Moreover,  the  mulch  should  be  renewed  after 
every  rain  and  should,  except  in  special  cases,  be  not  more 


THE  CONTROL  OF  SOIL-MOISTURE  219 

than  three  inches  deep.  Late  in  the  season,  especially  for 
corn,  the  cultivation  should  be  shallow  to  prevent  root-prun- 
ing.1 

For  many  years  cultivation  for  a  soil-mulch  has  been  ad- 
vocated for  two  reasons:  (1)  checking  of  evaporation,  and  (2) 
the  killing  of  weeds.  Either  procedure,  if  successful,  will 
allow  the  crop  a  larger  proportion  of  the  rainfall.  Recent 
experimental  results,  however,  seem  to  indicate  that  a  soil- 
mulch  with  an  intertilled  crop  does  not  check  evaporation 
compared  with  a  soil  uncultivated  and  kept  free  of  weeds. 
This  is  probably  due  to  the  fact,  that  even  with  moisture 
a  limiting  factor,  the  water  sacrificed  in  renewing  the  mulch 
is  not  offset  by  that  conserved.  The  tendency  of  soils,  espe- 
cially those  of  a  sandy  character  to  self -mulch  as  well  as  the 
action  of  the  roots  of  the  crop  in  intercepting  the  water,  may 
also  be  factors.  Under  greenhouse  conditions  and  in  regions 
of  very  little  rainfall,  the  soil-mulch  probably  does  conserve 

1  Since  a  great  many  of  the  inter-tilled  crops  are  shallow-rooted, 
great  care  should  be  exercised  in  cultivation,  especially  toward  the  latter 
part  of  the  growing  season.  Corn  and  potatoes  are  especially  influenced 
by  root-pruning.  The  following  data 2  averaged  for  7  years  are 
pertinent : 

Influence  of  Boot-Pruning  on  the  Yield  of  Corn  in  Bushels  to 

the  Acre.    Average  of  7  Years.    University 

of  Illinois 


Treatment 


No  cultivation,  weeds  kept  down  with  hoe. 

Shallow  cultivation 

Deep  cultivation , 


Shallow  cultivation,  roots  unpruned 

Shallow  cultivation,  roots  pruned  with  knife. 


Surface  scraped,  roots  unpruned 

Surface  scraped,  roots  pruned  with  knife. 


Yield 


67.7 

70.8 
68.6 

74.8 
61.6 

80.7 
68.3 


, 


2  Mosier,  J.  G.,  and  Gustaf son,  A.  F.,  Soil  Moisture  and  Tillage  for 
Corn;  111.  Agr.  Exp.  Sta.,  Bui.  181,  1915. 


220        NATURE  AND  PROPERTIES  OF  SOILS 

moisture.     The  following  figures  ■  are  representative  of  the 
data  available  regarding  these  points : 

Table  XLIV 

MOISTURE    CONTENT    OF    BARE    IRRIGATED    AND    DRY-LAND    PLOTS, 

TREATED  IN  VARIOUS  WAYS,  EXPRESSED  IN  TOTAL  INCHES  OP 

WATER   IN   UPPER  6  FEET   OF  SOIL.      GARDEN   CITY, 

KANSAS,   1914. 


Irrigated 

Dry  Land 

Treatment 

mar.  30 

SEPT.  16 

GAIN  OR 
LOSS 

mar.  30 

SEPT.  16 

GAIN  OR 
LOSS 

6-inch  mulch .... 

3-inch  mulch 

Bare  surface 

Weeds 

17.6 
18.1 
17.8 
16.4 

15.9 

16.6 

15.6 

9.1 

—1.7 
—1.5 
—2.2 
—7.3 

11.8 
11.3 
11.5 
10.8 

12.4 

11.7 

12.0 

8.0 

+  -6 

+  -4 
+  -5 
—2.8 

Table  XLV 

EFFECTS  OF  VARIOUS  METHODS  OF  TILLAGE  ON  THE  YIELD  OF  CORN 

AND  THE  AVERAGE  PERCENTAGE  OF  MOISTURE  IN  THE  SOIL 

TO  A  DEPTH  OF  40  INCHES.      AVERAGE  OF  8  YEARS ' 

TEST    AT    THE    UNIVERSITY    OF    ILLINOIS.2 

MEAN     RAINFALL,      33.7      INCHES. 


Treatment 

Yield  of  Corn 
Bushels 
Per  Acre 

Average 

Percentage 

op  Moisture 

in  Soil 

Not  plowed  or  cultivated : 

Kept   bare   of   weeds   only .... 
Plowed  and  seedbed  prepared : 

Kept  bare  of  weeds  only 

Weeds  allowed  to  grow 

Three  shallow  cultivations .... 

31.4 

45.9 

7.3 

39.2 

23.1 

22.3 

21.8 
21.9 

1Call,  L.  E.,  and  Sewell,  M.  C,  The  Soil  Mulch;  Jour.  Amer.  Soc. 
Agron.,  Vol.  9,  No.  2,  pp.  49-61,  Feb.,  1917. 

aMosier,  J.  G.,  and  Gustafson,  A.  F.,  Soil  Moisture  and  Tillage  for 
Corn;  111.  Agr.  Exp.  Sta.,  Bui.  181,  Apr.,  1915. 


THE  CONTROL  OF  SOIL-MOISTURE  221 

The  above  data,  which  are  amply  corroborated  by  other 
investigations,1  indicate  that,  with  an  uncropped  light  silt 
loam  in  a  semi-arid  region,  the  soil-mulch  is  of  little  practical 
importance  in  conserving  moisture.  Moreover,  the  results  in 
Illinois  as  well  as  Kansas  are  no  better  on  cropped  land,  the 
cultivation  seemingly  having  little  influence  on  either  mois- 
ture content  or  crop  yield.  The  importance  of  a  good  seed- 
bed is  very  strikingly  shown  by  the  Illinois  data,  as  is  also  the 
necessity  of  weed  control.  The  weeds  not  only  appropriate 
moisture  that  should  go  to  the  crop,  but  at  the  same  time 
absorb  nutrients  that  should  be  utilized  in  other  ways. 

Certain  general  conclusions  are  unavoidable  in  respect  to 
a  soil-mulch.2  In  the  first  place,  a  cropped  cultivated  soil 
seems  no  more  effective  in  preventing  evaporation  than  one 
that  is  cropped  and  uncultivated.  Whether  this  extends  to 
bare  soil  under  all  conditions  has  not  been  conclusively  shown. 
In  the  second  place,  the  elimination  of  weeds  seems  to  be  the 
most  important  benefit  of  cultivation.  It  must  be  remem- 
bered, however,  that  cultivation  may  exert  some  benefit  on 
aeration  of  a  heavy  soil  and  certainly  encourages  granulation 
to  a  certain  extent. 

114.  Summary  of  moisture  control. — Moisture  control 
seems  to  fall  logically  under  three  heads:  (1)  run-off,  (2) 
drainage,  and  (3)  evaporation.  The  detrimental  influence  of 
run-off  over  the  surface  is  due  to  erosion,  the  loss  of  the  water 

1  Young,  H.  J.,  The  Soil  Mulch;  Nebr.  Agr.  Exp.  Sta.,  25th  Ann. 
Eep.,  pp.  124-128,  1912. 

Barker,  P.  B.,  The  Moisture  Content  of  Field  Soils  Under  Different 
Treatments;  Nebr.  Agr.  Exp.  Sta.,  25th  Ann.  Eep.,  pp.  106-110,  1912". 

Cates,  J.  S.,  and  Cox,  H.  R.,  The  Weed  Factor  in  the  Cultivation  of 
Corn;  U.  S.  Dept.  Agr.,  Bur.  Plant  Ind.,  Bui.  257,  1912. 

Alway,  F.  J.,  Studies  on  the  Eelation  of  the  Non-available  Water  of 
the  Soil  to  the  Hygroscopic  Coefficient ;  Nebr.  Agr.  Exp.  Sta.,  Res.  Bui. 
3,  1913. 

Burr,  W.  W.,  The  Storage  and  Use  of  Soil  Moisture;  Nebr.  Agr.  Exp. 
Sta.,  Res.  Bui.  5,  1914. 

2  See  Call,  L.  E.,  and  Sewell,  M.  C,  The  Soil  Mulch;  Jour.  Amer. 
Soc.  Agron.,  Vol.  9,  No.  2,  pp.  49-61,  Feb.,  1917. 


222        NATURE  AND  PROPERTIES  OF  SOILS 

itself  being  of  minor  importance.  Similarly  percolation  loss 
is  important  because  of  the  nutrients  carried  away,  rather 
than  because  of  the  waste  of  the  water.  Since  a  certain 
amount  of  percolation  must  take  place  and  because  a  water- 
logged soil  is  unsanitary  for  plants,  rapid  drainage  is  essen- 
tial. Of  the  various  methods  available,  tile  drainage  is  the 
most  satisfactory.  Evaporation  loss,  as  with  run-off  and  per- 
colation, can  be  but  very  slightly  checked.  The  soil-mulch  is 
important  in  that  the  cultivation  necessary  to  produce  it  keeps 
down  the  weeds  and  in  this  manner  it  eliminates  serious  crop 
competition  for  nutrients  and  moisture. 


CHAPTER  XI 
SOIL  HEAT1 

It  is  universally  recognized  that  biological  activity  is  an 
energy  expression  and  that  such  activity  will  not  continue 
unless  certain  temperature  relations  are  maintained.  With 
higher  plants  this  heat  relation  has  two  phases,  the  tempera- 
ture of  the  air  and  that  of  the  soil.  The  former  is  clearly 
a  climatic  factor  and,  except  on  a  small  scale,  is  beyond  the 
control  of  man.  The  temperature  of  the  soil,  in  a  similar  way, 
is  subject  to  no  radical  regulation,  yet  soil  management  meth- 
ods provide  means  whereby  certain  small  but  biologically  vital 
modifications  can  be  made,  climatically  unimportant  but  prac- 
tically worthy  of  careful  consideration. 

115.  Importance  of  soil  heat. — Normal  plant  growth  is 
practically  suspended  at  a  temperature  of  40°  F.,  while  the 
germination  of  most  seeds  does  not  take  place  even  at  this 
point.  In  general,  it  is  poor  practice  to  place  certain  seeds 
and  plants  in  soil  where  growth  activities  will  not  occur  at 
once,  since  bacteria  and  fungi,  active  at  low  temperatures, 
may  sap  their  vitality  and  ultimately  cause  their  destruction. 
Three  groupings  of  higher  plants  may  be  made  as  far  as  their 
temperature  relationships  are  concerned.  Wheat  represents 
the  crops  that  germinate  and  grow  at  relatively  low  tempera- 
tures. Maize  requires  a  medium  temperature  for  proper 
growth,  while  pumpkins  and  melons  typify  crops,  the  heat 
requirements  of  which  are  very  high.     The  following  data 

1  For  a  bibliography  of  the  literature  of  soil  heat  see  Bouyoucos, 
G.  J.,  An  Investigation  of  Soil  Temperature  and  Some  of  the  Most 
Important  Factors  Influencing  It;  Mich.  Agr.  Exp.  Sta.,  Tech.  Bui.  17, 
pp.  194-196;  1913. 

223 


224        NATURE  AND  PROPERTIES  OF  SOILS 

from  Haberlandt1  show  the  need  of  careful  temperature  con- 
trol in  the  propagation  of  plants: 

Table  XL VI 

GERMINATION   TEMPERATURES 


Crop 

Minimum 

Optimum 

Maximum 

Wheat 

40°  F 

49 

52 

84°  F 

93 

93 

108°  F 

Maize 

Pumpkin 

115 
115 

Table  XL VII 

GROWTH   TEMPERATURES 

Crop 

Minimum 

Optimum 

Maximum 

Wheat 

32-40°  F 

40-51 

51-60 

77-  78°  F 
88-  98 
98-111 

88-  98 

Maize 

Pumpkin 

98-111 
111-122 

Other  desirable  biological  activities,  especially  those  due  to 
bacteria,  are  impeded  if  not  brought  entirely  to  a  standstill  by 
a  temperature  of  32°  F.  Such  changes  as  decomposition  of 
organic  matter,  the  production  of  ammonia  from  nitrogenous 
organic  matter,  the  formation  of  nitrate  nitrogen  from  am- 
monia and  the  fixation  of  atmospheric  nitrogen  depend  on 
heat  conditions  which,  fortunately,  are  optimum  for  the  de- 
velopment of  higher  plants. 

Desirable  chemical  reactions  in  the  soil  are  much  retarded 
by  low  temperatures,  heat  greatly  accelerating  such  phe- 
nomena. This  is  especially  noticeable  in  the  tropics  where 
weathering  is  much  more  rapid  and  intense  than  in  temperate 
regions.  Much  of  the  hydration,  oxidation,  carbonation  and 
solution  in  a  temperate  climate  occurs  in  the  summer  when 
high  temperature  lends  its  aid  to  such  desirable  reactions. 

1  Haberlandt,  F.,  Die  Oberen  und  Unteren  Temperaturgrenze  fur  die 
Keimung  der  Wichtigeren  Landwirthschaftlichen  Samereien;  Landw. 
Versuchs.  Stat.,  Band  17,  Seite  104-106,  1874. 


SOIL  HEAT  225 

The  effect  of  heat  on  the  physical  changes  within  the  soil 
is  often  vital.  The  influence  that  temperature  variation  exerts 
on  percolation,  evaporation,  and  capillary  movement  of  soil- 
water;  on  diffusion  of  gases,  vapors,  and  salts  in  solution; 
and  on  osmosis,  surface  tension  and  vapor  tension  phenomena, 
may  serve  as  examples  of  such  heat  modifications.  Moreover, 
successive  freezing  and  thawing  of  the  soil  greatly  aids  in 
granulation  and  aeration.  The  aspirating  effect  of  a  slight 
change  in  temperature  is  so  tremendous  as  often  markedly  to 
renew  the  oxygen  supply  of  the  furrow  slice. 

In  order  fully  to  understand  the  practical  and  scientific 
relationships  involved  in  even  a  partial  control  of  soil  heat, 
a  certain  cycle  of  events  must  be  recognized.  The  cycle  be- 
gins with  the  acquisition  of  energy  from  the  sun  and  the 
establishment  of  certain  temperature  relations  which  depend 
on  absorption  activity  and  the  facility  with  which  heat  is 
transferred  from  place  to  place.  The  important  chemical, 
physical,  and  biological  transformations  within  the  soil  de- 
pend as  much  on  such  movements  as  on  the  intensity  of  the 
temperature  factors.  Much  of  the  energy  so  involved  is  soon 
lost  from  the  soil,  returning  again  to  the  space  from  which 
it  came.  Thus  the  cycle  is  completed,  having  provided  the 
temperature  conditions  necessary  for  successful  crop  produc- 
tion.    (See  Fig.  41.) 

116.  Insolation  received  by  the  soil. — The  sun  supplies 
practically  all  of  the  energy  by  means  of  which  the  soil  main- 
tains a  temperature  suitable  for  its  normal  activities.  Energy 
from  other  sources  is  negligible.  Radiation,  the  means  by 
which  this  transfer  is  affected,  is  a  free  wave  movement  of 
some  type.  It  is  an  oscillatory  phenomenon,  the  space  between 
the  sun  and  the  receiving  body  being,  so  far  as  is  known,  en- 
tirely unaffected.    The  length1  of  such  oscillations  varies  from 

1  The  approximate  wave  lengths  are  as  follows : 

Infra-red    000270  to  .000075  cm. 

Light  waves 000075  to  .000036  cm. 

Ultra-violet   000036  to  .000019  cm. 


226        NATURE  AND  PROPERTIES  OF  SOILS 


the  short  ultra-violet  rays,  through  the  so-called  light  wave 
series  to  the  long  infra-red  rays,  the  latter  possessing  the 
greatest  heat  possibilities.  The  insolation  of  energy  received 
at  the  upper  limits  of  the  earth's  atmosphere  varies  with  the 
season  and  with  the  position  chosen.1 

Due  to  the  gases  of  the  atmosphere  and  especially  to  clouds 
and  dust,  only  a  small  portion  of  the  total  insolation  actually 


ATMOSPHERE 


RAD/ANT 
ENERGY 

ABSORPTION 

REFLECTION 

REFRACT/OR, 


EVAPORATION 


RADIATION 


CONDUCTION 


REFLECTION 


SOIL 


CONDUCTION 
CONVECTION 
ORGANIC  DECAY 


/-ABSORPTION 
_  COLOR, 5LORE 
2- SPECIFIC  HEAT 
TEXTURE 
STRUCTURE 
MOISTURE 
3- MOVEMENT 
A- LOSS  OF  HEAT 


rise  or 

TEMPERATURE 


Fig.  41. — A  diagram  showing  the  heat  relations  of  soil. 

does  work  either  on  the  land  or  water  surfaces  of  the  earth. 
The  atmosphere  and  its  impurities  probably  deflect  on  an 
average  more  than  three-fourths  of  the  insolation  by  absorp- 
tion, reflection,  and  refraction.  Little  or  none  of  .such  energy 
ever  reaches  the  earth  itself.  Clouds  and  dust  play  an  im- 
portant role  in  such  interception,  affecting  to  a  marked  degree 
the  energy  received  at  any  particular  location.  Part  of  the 
original  insolation  reaching  the  earth 's  surface  is  immediately 
reflected  and  is  lost  as  radiant  energy,  having  undergone  no 

1  The  earth  and  its  atmosphere  receives  but  one  two-billionth  of  the 
sun's  energy.  Cm  such  a  trifling  proportion  of  the  sun's  energy  depend 
almost  all  of  the  earth's  activities. 


SOIL  HEAT  227 

transformation  and,  therefore,  having  done  no  work.  This 
reflection  is  much  greater  on  sea  than  on  land  and  greater 
from  snow  than  from  soil  surfaces.  Reflection  is  influenced 
to  a  marked  degree  by  vegetation,  stubble,  for  example,  being 
more  effective  than  a  green  field,  a  forest  or  even  bare  soil. 
Possibly  one-fifth  of  the  earth's  insolation  on  the  average  is 
absorbed  by  the  land  and  water  surfaces,  being  the  source  of 
the  energy  which  later  functions  both  statically  and  dynam- 
ically in  the  soil. 

The  statement  is  often  made  that  warm  rain  carries  con- 
siderable heat  into  the  soil.  Such  an  assertion  is  not  only 
misleading  but  in  most  cases  entirely  incorrect.  Precipitation 
in  general  is  usually  cooler  than  the  soil  in  temperate  regions, 
especially  in  the  summer.  Rain  is  spoken  of  as  warm,  not 
in  comparison  with  soil  but  with  average  rain-water  tempera- 
ture. Even  if  rain  water  should  be  10°  F.  warmer  than  the 
soil,  a  very  improbable  assumption,  an  average  rain  would 
raise  the  temperature  of  the  surface  six  inches  only  slightly. 

117.  Absorption  of  insolation. — The  energy  received 
from  the  sun  functions  in  a  number  of  ways  on  reaching  the 
land  surfaces  of  the  earth.  It  may  accelerate  chemical  re- 
actions, it  may  be  absorbed  by  plants,  it  may  induce  certain 
changes  in  form  and,  lastly,  it  may  be  converted  into  heat. 
It  is  in  this  latter  state  that  insolation  energy  plays  its  most 
important  part  in  soil  activities,  since  heat  energy  may  act 
in  ways  that  radiant  energy  finds  impossible.  Since  heat  is 
commonly  conceived  as  the  kinetic  energy  of  the  molecules 
of  a  body,  it  is  quite  distinct  and  different  from  solar  radia- 
tion, which  must  encounter  some  favorable  substance  before 
heat  is  produced.  Temperature  is  the  condition  of  a  body 
in  respect  to  its  heat  energy  and  is  the  common  mode  of  ex- 
pressing heat  intensity.1 

1  Molecules  are  in  constant  motion,  colliding  with  their  neighbors,  re- 
bounding, and  quivering.  They  possess  energy  which  is  called  heat. 
Temperature  is  determined  by  the  velocity  of  the  molecules  and  is  a 
manifestation  of  heat. 


228        NATURE  AND  PROPERTIES  OF  SOILS 

Certain  inherent  qualities  of  the  soil  as  well  as  its  position 
tend  to  influence  its  capacity  to  absorb  radiant  energy.  The 
effect  may  be  measured  in  the  resultant  rise  in  temperature, 
providing  all  variables  are  under  control.  The  factors  in- 
volved are  texture,  structure,  color,  and  position.  Only  the 
last  two  are  of  practical  importance. 

118.  Influence  of  color  on  absorption. — It  is  well  known 
that  a  black  surface  absorbs  more  energy  than  a  white 
one  under  similar  conditions  and  will  register  a  more  rapid 
and  a  higher  temperature  rise.  This  is  because  of  a  difference 
in  reflection,  the  white  surface  being  more  effective  in  this 
respect.  The  same  principle  has  been  shown  by  a  number 
of  investigators  to  hold  with  soil.1  The  addition  of  organic 
matter,  provided  its  decomposition  has  been  of  the  proper 
sort,  will,  other  factors  being  equal,  favor  a  higher  soil  tem- 
perature. Wollny2  in  experimenting  with  soil  covered  with 
thin  layers  of  different  colored  material  obtained  some  inter- 
esting field  data.  The  black  soil  not  only  exhibited  the  high- 
est temperature  but  also  showed  the  greatest  fluctuation.  Min- 
imum temperatures  were  the  same  regardless  of  color,  while 
temperature  differences  decreased  with  depth.  The  curves 
in  Fig.  42  are  typical  of  Wollny 's  results  on  clear  days. 

Besides  the  quite  obvious  effect  of  color  on  rate  of  energy 
absorption,  the  curves  exhibit  two  other  points  worthy  of 
notice.  The  first  is  the  tendency  of  the  soil  temperature  to 
lag  behind  that  of  the  air  and  the  second  is  the  equal  minima 
reached  by  the  two  soils.  The  latter  tendency  would  seem 
to  indicate  that  color  has  little  effect  on  the  radiation  of  heat 
by  soil. 

1  Bouyoucos,  G.  J.,  An  Investigation  of  Soil  Temperature;  Mich.  Agr. 
Exp.  Sta.,  Tech.  Bui.  17,  p.  30,  1913. 

Lang,  C.,  Voer  Warme-ab sorption  und  Emission  des  Boden;  Forsch. 
a.  d.  Gebiete  d.  Agr.-Physik.,  Band  I,  Seite  379-407,  1878. 

3  Wollny,  E.,  Untersuchung  uber  den  Einfluss  der  Farbe  des  Bodens 
auf  dessen  Erwarmung ;  Forsch.  a.  d.  Gebiete  d.  Agri.-Physik.,  Band  I, 
Seite  43-69,  1878.  Also,  Untersuchung  en  uber  den  Einfluss  der  Farbe 
des  Bordens  auf  dessen  Erwarmung ;  Forsch.  a.  a.  Gebiete  d.  Agr.-Phys., 
Band  IV,  Seite  327-365,  1881. 


SOIL  HEAT 


229 


119.  The  effect  of  slope  on  absorption. — The  second 
phase  to  be  considered  in  the  rise  of  temperature  of  a  given 
soil  is  the  angle  of  incident  of  the  sun's  rays.  The  greater 
the  inclination  of  a  soil  from  a  right  angle  interception,  the 
less  rapid  will  be  the  rise  in  temperature.  As  a  consequence, 
the  total  insolation  received  in  the  tropics  to  a  unit  area  is 
greater  than  that  attained  by  a  corresponding  area  in  the 
temperate  zone.     Moreover,  any  condition  in  a  temperature 


Fig.  42. — Curves  showing  the  temperature  variations  of  different  colored 
soils  at  a  four  inch  depth  compared  with  air  temperature.  Munich, 
June  23,  1876. 


region  which  tends  to  bring  a  unit  surface  more  nearly  normal 
to  the  sun's  rays  will  increase  its  absorbed  energy  and  raise 
its  average  seasonal  temperature.  In  the  north  temperate 
zone  this  is  of  course  a  southerly  inclination.  The  diagram 
(Fig.  43)  illustrating  conditions  on  the  42d  parallel  at  noon 
on  June  21  makes  clear  this  relationship. 

It  is  seen  that  in  this  case  a  southerly  slope  of  20°  received 
the  greatest  amount  of  heat  to  a  unit  area  with  the  level  soil 


230        NATURE  AND  PROPERTIES  OF  SOILS 

next  and  the  northerly  slope  last.     The  amount  of  heat  for 
a  given  area  is  in  the  order  of  106,  100,  and  81,  respectively. 


Fig.  43. — Diagram  showing  the  distribution  of  a  given  amount  of  radiant 
energy  on  different  slopes  on  June  21,  at  the  42nd  parallel  north. 


These  generalizations  have  been  established  by  the  work  of 
a  number  of  investigators.1 

Wollny2  found  near  Munich  that  the  temperature  of  south- 

1King,  F.  H.,  Physics  of  Agriculture,  p.  218.     Madison,  Wis.,  1910. 
a  Wollny,  E.,  Untersuchungen  uber  den  Einfluss  der  Exposition  auf  die 


SOIL  HEAT  231 

ward  slopes  varied  with  the  time  of  year.  For  example,  the 
southeasterly  inclination  was  warmest  in  the  early  season,  the 
southerly  slope  during  mid-season  and  the  southwesterly  slope 
in  the  fall.  Such  a  relationship  is  of  course  governed  entirely 
by  local  climatic  conditions,  especially  cloudiness,  and  might 
not  be  true  of  any  other  place.  A  southeasterly  slope  is  gen- 
erally preferred  by  gardeners.  Orchardists  also  pay  strict 
attention  to  the  aspect  as  it  is  often  a  factor  in  sun-scald  and 
certain  plant  diseases. 

120.  Rise  of  temperature  and  the  factors  involved. — 
The  rise  of  temperature  of  a  layer  of  soil  following  a  given 
absorption,  depends  (1)  on  the  specific  heat  of  the  soil,  (2)  on 
the  rate  at  which  the  heat  moves  to  other  parts  of  the  soil 
mass,  and  (3)  on  the  losses  of  heat  to  the  atmosphere.  It  is 
evident  that  in  a  study  of  the  influence  of  insolation  on  soil 
temperature,  specific  heat  should  receive  the  first  attention. 

121.  Specific  heat  and  soil  temperature. — The  specific 
heat  of  any  material  may  be  defined  as  its  thermal  capacity 
compared  with  that  of  water.  It  is  expressed  as  a  ratio  to  the 
quantity  of  heat  required  to  raise  the  temperature  of  a  given 
amount  of  a  certain  substance  1°  C.  to  the  quantity  needed 
to  change  an  equal  amount  of  water  from  15°  to  16°  C. 

The  specific  heat  figure  for  soil  generally  refers  to  the  heat 
capacity  of  the  dry  substance.  Under  normal  conditions,  soils 
contain  variable  amounts  of  pore  spaces  and  consequently 
have  different  weights  to  the  cubic  foot.  A  specific  heat  figure 
based  on  weight,  therefore,  does  not  give  a  true  idea  of  the 
relative  heat  capacities  of  two  soils.  The  expression  of  spe- 
cific heat  by  volume  seems  a  more  rational  basis  of  compari- 
son.1 The  specific  heat  of  the  soil  is  important  because  of  the 
relation  it  has  to  the  warming  up  of  soil  in  the  spring,  the 

Erwarmung  des  Bodens;  Forsch.  a.  d.  Gebiete  d.  Agr.-Physik.,  Band  I, 
Seite  263-294,  1878.  This  publication  contains  a  number  of  other  papers 
on  this  subject  by  Wollny. 

1  Weight  specific  heat  of  a  substance  may  be  expressed  by  the  number 
of  calories  required  to  raise  the  temperature  of  one  gram,  1°  C.    Volume 


232 


NATURE  AND  PROPERTIES  OF  SOILS 


rate  of  cooling  in  autumn,  drainage  influences,  and  like  phe- 
nomena. 

Specific  heat  data  from  different  investigators  do  not  show 
the  agreement  that  might  be  expected.1  This  is  probably  due 
(1)  to  inaccuracies  in  the  naming  of  the  soils  used,  (2)  to 
difference  in  methods,  and  (3)  to  difficulties  in  technique. 
Everything  considered,  the  following  table  from  Ulrich  2  dis- 
plays in  a  suitable  way  the  important  specific  heat  phases: 


Table  XLVII 

VOLUME  OF  SPECIFIC  HEAT  OF 

SOIL 

Soils 

Weight 
Volume 

Volume 
Specific  Heat 

Sand 

Clay 

1.52 

1.04 

.37 

.2901 
.2333 

Organic  matter 

.1639 

It  is  evident  that  specific  heat  is  partially  governed  by  the 
organic  matter  of  the  soil  and  partially  by  texture  and  struc- 

specific  heat  is  the  number  of  calories  necessary  to  raise  the  temperature 
of  one  cubic  centimeter  of  the  substance  one  degree.  In  the  case  of 
soil,  weight  specific  heat  may  be  changed  to  volume  specific  heat  by 
multiplying  it  by  the  volume  weight,  since  volume  weight  is  the  weight 
in  grams  of  one  cubic  centimeter  of  dry  soil. 

■*  The  following  weight  specific  heats  from  Lang,*  Patten  t  and  Bou- 
youcos  t  are  interesting : 


Lang 
Coarse  sand 198 


Patten 


Sand 


,185 


Limestone  soil. . .   .249      Sandy  loam .183 


Organic   soil 257 

Garden  soil 276 

Peat 477 


Loam   191 

Loam    194 

Clay 210 


Bouyoucos 

Sand    193 

Gravel 204 

Clay    206 

Loam 215 

Peat 252 


*  Lang,  C,  trber  Warme  Capacitat  der  Bodeneonstituenten;  Forsch.  a. 
d.  Gebiete  d.  Agr.-Phys.,  Band  I,  Seite  109-147,  1878. 

t  Patten,  H.  E.,  Beat  Transference  in  Soils;  U.  S.  Dept.  Agr.,  Bur. 
Soils,  Bui.  59,  p.  34,  1909. 

t  Bouyoucos,  G.  J.,  An  Investigation  of  Soil  Temperature ;  Mich.  Agr. 
Exp.  Sta.,  Tech.  Bui.  17,  p.  12,  1913. 

3  Ulrich,  K.,  TJntersuchungen  uber  Warmekapazitat  der  Bodenkonsti- 
tuenten;  Forsch.  a.  d.  Gebiete  d.  Agr.-Phys.,  Band  17,  Seite  1-31,  1894. 


SOIL  HEAT 


233 


ture.  Organic  matter  will  lighten  and  loosen  a  soil,  and  lower 
the  volume  weight.  Moreover,  its  heat  capacity  is  low.  The 
effect  of  such  an  addition  is  to  lower  the  specific  heat  figure. 
It  is  apparent  also  that  the  finer  the  texture  of  the  soil,  the 
lower  the  specific  heat.  That  is  due  not  to  a  difference  in 
chemical  composition  but  to  a  lowered  volume  weight.  Any 
practice,  therefore,  that  tends  to  vary  volume  weight  will  in 
a  like  manner  vary  specific  heat.  The  farmer  may  encourage 
the  warming  of  his  soil  by  deep  and  efficient  plowing.  By 
increasing  its  organic  content,  he  may  create  a  tendency  in 
the  same  direction. 

One  other  factor,  more  important  than  those  already  men- 
tioned, yet  remains  to  be  discussed.  This  is  water,  so  univer- 
sally present  in  soils  and  so  important  in  natural  soil  phe- 
nomena. As  the  specific  heat  of  water  is  several  times  greater 
than  that  of  the  soil  constituents,  any  addition  of  it  must  raise 
the  thermal  capacity  of  the  mass.  The  following  data  from 
IJlrich1  show  that  moisture  rather  than  texture  and  organic 
matter  is  the  controlling  factor  in  normal  soil: 


Table  XL VIII 

THE   EFFECT    OF    MOISTURE   ON   VOLUME   SPECIFIC    HEAT   OF   SOIL 
(moisture  expressed  as  a  percentage  of  the  total  water  capacity) 


Dry 
Soil 

10% 
Water 

20% 
Water 

40% 
Water 

60% 
Water 

80% 
Water 

100% 
Water 

Sand 

.291 

.330 
.294 
.242 

.368 
.355 
.320 

.444 

.478 
.476 

.520 
.600 
.632 

.597 
.723 

.788 

.675 

Clay 

Organic  matter 

.233 
.164 

.845 
.945 

The  overwhelming  influence  of  moisture  is  at  once  evident 
from  these  data.  Fine  texture,  because  of  its  high  water 
capacity,  usually  accentuates  the  dominance  of  moisture. 
Organic  matter  functions  in  the  same  way.     While  an  organic 

1  Ulrieh,  E.,  Untersuchungen  uber  die  W armekapazitat  der  BodenTconsti- 
tuenten;  Forsch.  a.  d.  Gebiete  d.  Agr.-Phys.,  Band  17,  Seite  27,  1894. 


234        NATURE  AND  PROPERTIES  OF  SOILS 

soil  of  low  volume  weight  may  warm  up  easily  when  dry,  its 
high  water  content  usually  markedly  retards  its  temperature 
change.  A  muck  soil  is  usually  the  last  to  freeze  in  winter 
and,  conversely,  the  last  to  thaw  in  spring.  The  advantage 
of  drainage  is  evident  as  a  wet  soil  is  of  necessity  colder  in 
the  spring  than  one  that  is  well  drained.  This  at  least  par- 
tially accounts  for  the  fact  that  a  sandy  soil  is  usually  an  early 
one  and  is,  therefore,  of  particular  value  in  trucking. 

122.  Heat  movements  in  soil. — While  volume  weight,  or- 
ganic matter,  and  moisture  seem  largely  to  control  the  degree 
to  which  a  soil  will  become  heated  when  exposed  to  insolation,  * 
it  is  evident  that  there  must  be  some  mode  of  energy  transfer 
whereby  such  phenomena  may  be  facilitated.  Heat  movement 
is  necessary  in  order  that  the  lower  layers  of  the  soil  may 
become  warm  enough  for  proper  biological  functionings. 
Energy  transmission  both  downward  and  laterally  is  abso- 
lutely essential  and  deserves  as  much  attention  as  the  factors 
influencing  insolation  absorption. 

Two  methods  of  heat  transfer  function  in  a  normal  soil — 
conduction  and  convection.  These  modes  of  energy  move- 
ment are  extremely  difficult  to  analyze,  due  to  the  impossi- 
bility of  controlling  one  while  studying  the  other. 

123.  Conduction  of  heat  in  soil. — While  radiation  has  to 
do  with  the  oscillatory  transfer  of  energy  conduction  relates  to 
the  molecular  transmission  of  heat  through  any  material. 
When  one  part  of  a  substance  is  heated,  the  movement  of  its 
molecules  is  stimulated.  These  molecules  strike  their  neighbors 
with  increased  force,  thus  quickening  their  motion.  These  in 
turn  accelerate  others  until  the  energy  applied  at  one  point 
becomes  apparent  at  another.  Solids  as  a  class  are  better  con- 
ductors than  liquids,  while  liquids  in  general  are  superior  to 
gases  in  this  respect.  It  must  be  remembered  in  studying  the 
conductivity  of  heat  through  soil,  that  we  are  dealing  with 
a  heterogeneous  mixture  of  mineral  and  organic  matter  con- 
taining varying  amounts  of  air  and  water.     The  movement 


SOIL  HEAT  235 

of  soil  heat  involves  not  only  the  question  of  conduction 
through  solids  but  through  liquids  and  gases  as  well.  More- 
over, transfer  resistance,  which  occurs  at  the  boundary  of  two 
substances  in  contact,  has  much  to  do  with  the  rate  of  trans- 
mission. In  addition,  the  air  and  water  of  the  soil  are  capable 
of  considerable  movement  which  makes  conductivity  studies 
extremely  difficult  due  to  convection  currents. 

The  heat  conductivity  of  soil  is  affected  by  a  number  of 
factors  which  may  or  may  not  lend  themselves  to  field  con- 
trol. Important  among  these  are  texture,  structure,  organic 
matter,  and  moisture.  The  influence  of  the  first  is  clearly 
shown  by  the  following  comparative  data  obtained  by  Bou- 
youcos,1  with  field  soils: 

Table  XLIX 

RELATIVE   CONDUCTIVITY   AS   MEASURED   BY   THE   TIME   REQUIRED 

FOR  A  THERMOMETER  7  INCHES  FROM  THE  SOURCE  OF  HEAT 

TO  INDICATE  A  RISE  IN  TEMPERATURE 


These  results  are  comparative  only  in  a  qualitative  way. 
Quantitative  determinations  are  so  beset  by  error  that  only 
few  investigators  have  made  any  consistent  attempt  along  this 
line.    Patten's  results 2  expressed  as  metric  K  3  (the  heat  con- 

1  Bouyoucos,  G.  J.,  An  Investigation  of  Soil  Temperature;  Mich.  Agr. 
Exp.  Sta.,  Tech.  Bui.  17,  p.  20,  1913. 

3  Patten,  H.  E.,  Heat  Transfer  in  Soils;  U.  S.  Dept.  Agr.,  Bur.  Soils, 
Bui.  59,  p.  26-28,  1909. 

3  The  conductivity  of  a  substance  is  measured  by  the  number  of  gram- 
calories  of  heat  transmitted  in  1  second  through  a  cube  with  1  centi- 
meter edges,  when  the  opposite  faces  differ  in  temperature  by  1°C.  The 
calories  of  heat  transmitted  (H)  will  be  proportional  to  the  area  of  the 


236        NATURE  AND  PROPERTIES  OF  SOILS 

ductivity  coefficient  in  C.G.S.  units)  shows  the  same  general 
comparisons  as  already  presented: 

Table  L 

CONDUCTIVITY    COEFFICIENTS   OF   DIFFERENT   DRY    SOILS 


Soils 

K 

Coarse  quartz 

.000917 

Leonardtown  loam 

.000882 

Podunk  fine  sandy  loam 

.000792 

Hagerstown  loam 

Galveston  clay 

.000699 
.000577 

Muck 

.000349 

It  is  evident,  in  general,  that  the  finer  the  texture  of  the 
soil,  the  lower  is  the  conductivity.  This  cannot  be  construed 
as  indicating  that  the  conductivity  coefficients  of  sand  and 
clay  particles  are  particularly  different.  The  variance  ob- 
served is  adequately  explained  by  the  great  number  of  trans- 
fers necessary  in  a  fine-textured  soil.  It  is  also  evident  that 
the  addition  of  organic  matter  will  lower  conductivity. 
Humus  itself  has  a  low  conductivity  coefficient  and  would 
markedly  affect  the  transfer  resistance  by  changing  the  struc- 
ture of  the  soil.  Compacting  a  soil  should  accelerate  heat 
transfer  due  to  a  more  intimate  contact  of  the  soil  grains  and 
a  consequent  diminution  of  transfer  interference.  Tillage, 
on  the  contrary,  must  impede  not  only  the  movement  of  heat 
downward  in  the  soil  but  from  the  subsoil  into  the  furrow 
slice. 

The  greatest  single  factor  to  be  considered  in  heat  conduc- 
tivity is  the  moisture  content  of  the  soil.    The  curve  (Fig.  44) 

faces  (A)  and  to  the  differences  in  temperature  of  the  faces  (f — t"), 
while  it  will  be  inversely  proportional  to  the  thickness  (d)  of  the  cube. 
K  is  a  constant,  depending  on  the  material  studied. 

H=;K.A(t--t"), 
d 


SOIL  HEAT 


237 


for  fine  sandy  loam,  constructed  from  Patten's  data,1  illus- 
trates its  effect  and  indicates  how  heavily  it  must  override  the 
factors  already  mentioned: 


,00300 

.00400 
.00300 

I 

.00200 

St 

I 

.OOIOO 

8 

0 

PEHCEm 

r   OF  MC 

ilSTURE 

IN  SOJL 

0  5  to  15  20  25 

Fig.  44. — Conductivity  curve  for  Podunk  fine  sandy  loam,  showing  the 
influence  of  moisture  content  upon  the  rate  of  heat  transfer.  The 
curve  apparently  flattens  out  at  a  high  moisture  content  indicating 
that  good  conductivity  may  he  obtained  at  optimum  moisture. 

At  first  glance  it  appears  peculiar  that  the  heat  movement 
through  a  soil,  the  mineral  constituents  of  which  possess  a 
conductivity  coefficient  of  about  .01066,  should  be  accelerated 
by  the  addition  of  a  liquid  possessing  a  value  for  K  of  about 
.00149.     The  explanation  lies  in  the  lowering  of  the  transfer 

1  Patten,  H.  E.,  Heat  Transfer  in  Soils;  U.  S.  Dept.  Agr.,  Bur.  Soils, 
Bull.  59,  p.  27,  1909. 


238        NATURE  AND  PROPERTIES  OF  SOILS 

resistance.  Heat  passes  from  soil  to  water  about  150  times 
easier  than  from  soil  to  air.  As  the  water  increases,  the  air 
decreases  and  the  rate  of  conductivity  is  raised.  When  suf- 
ficient water  is  present  to  join  all  of  the  soil  particles,  further 
additions  will  have  little  effect  on  character  of  heat  movement. 
Moisture,  optimum  for  crop  growth,  amply  provides  for  heat 
transfer.  The  slow  warming  up  of  the  lower  subsoil  must  not 
be  taken  as  an  indication  of  lower  conductivity.  It  is  due 
rather  to  a  lessened  heat  supply.  As  a  matter  of  fact,  the 
rate  of  heat  transmission  has  been  shown  to  be  more  rapid  in 
the  subsoil,  due  to  a  greater  compaction  and  to  the  presence 
of  more  water. 

This  brief  discussion  of  conductivity  shows  the  vital  im- 
portance of  such  a  phenomenon  to  plants  in  that  the  necessary 
heat  is  carried  broadcast  through  the  soil.  While  conduc- 
tivity is  affected  to  a  certain  extent  by  texture,  structure,  and 
organic  matter,  moisture  is  the  dominant  factor.  Under  nat- 
ural conditions,  it  is  necessary  to  maintain  a  medium  amount 
of  water  in  the  soil.  This  moisture  condition,  fortunately, 
supports  almost  maximum  heat  conduction.  Good  tilth  and 
increased  organic  matter  probably  exert  their  greatest  in- 
fluence on  this  type  of  heat  transfer  by  their  influence  on  soil 
moisture. 

124.  Convection  transfer  of  heat. — Convection,  the  third 
manner  by  which  energy  may  be  conveyed,  is  a  heat  transfer 
by  means  of  currents  in  liquids  or  gases.  It  functions  by  an 
actual  and  obvious  movement  of  matter.  In  the  soil  absorp- 
tion tends  to  heat  the  air  as  well  as  the  solid  substance.  This 
produces  currents  due  to  the  expansion  and  rise  of  the  warmed 
gases.  It  is  obvious  that  such  heat  movement  must  always  be 
lateral  or  upward,  never  downward.  Such  convection  exerts 
its  greatest  influence  in  equalizing  the  temperature  of  the  soil, 
overcoming  the  effects  of  unequal  conduction  and  uneven  ab- 
sorption due  to  vegetation  or  stone.  Air  currents  as  they 
escape  into  the  upper  air  carry  considerable  heat  away  from 


SOIL  HEAT  239 

the  soil.  Such  a  loss  is  of  little  moment,  however,  compared  to 
that  continually  occurring  through  conduction  and  radiation. 

Some  heat  is  carried  downward  into  the  soil  by  percolat- 
ing water.  This  is  a  true  convection  activity.  The  impor- 
tance of  such  a  heat  transfer  is  only  conjectural.  As  percola- 
tion is  generally  intermittent  in  a  soil,  it  is  probable  that  it 
does  not  modify  to  any  extent  the  influence  exerted  by  con- 
duction. 

125.  Effect  of  organic  matter  on  soil  temperature. — 
Plants  entrap  a  considerable  amount  of  radiant  energy  from 
the  sun,  part  of  which  is  utilized  during  the  growth  period. 
The  remainder  exists  as  latent  energy  in  the  tissue.  If  any 
amount  of  plant  remains  are  incorporated  in  the  soil  and  de- 
cay proceeds,  this  heat  is  liberated.  Thus  a  heat  transfer  is 
similar  in  a  way  to  convection,  except  that,  in  this  case,  the 
transfer  is  by  the  movement  of  a  solid  and  the  energy  is 
latent. 

To  what  extent  the  decay  of  organic  matter  is  effective  in 
bringing  about  any  important  modification  of  field  soil,  it  is 
difficult  to  say.  In  greenhouses  and  hotbeds  perceptible  in- 
creases are  obtained  by  the  use  of  fresh  manure.  In  the  field, 
however,  where  the  absorption  and  loss  of  heat  are  very  large 
and  where  the  organic  matter  makes  up  but  a  small  portion 
of  the  soil  mass,  it  is  doubtful  whether  any  important  heat 
increase  occurs.  Georgeson,1  in  Japan  during  the  first  twenty 
days  after  an  application  of  eighty  tons  of  manure  to  the 
acre,  obtained  an  increase  of  only  3.4°  F.  over  a  soil  un- 
treated. Wagner  2  found  an  average  increase  of  1°  F.  from 
the  use  of  twenty  tons  of  barnyard  manure  to  the  acre.  Bou- 
youcos  3  has  obtained  the  latest  data  on  the  subject.    Under 

1  Georgeson,  C.  C,  Influence  of  Manure  on  Soil  Temperature;  Agri. 
Sci.,  Vol.   1,  pp.  25-52,   1887. 

a  Wagner,  F.,  fiber  den  Einfluss  der  Dungung  mit  Organischen  Sub- 
stance auf  die  Bodentemperatur;  Forsch.  a.  d.  Gebiete  d.  Agr.-Phys., 
Band  V,  Seite  373-405,  1882. 

•Bouyoucos,  G.  J.,  An  Investigation  of  Soil  Temperature;  Mich.  Agr. 
Exp.  Sta.,  Tech.  Bui.  17,  pp.  180-190,  1913. 


240        NATURE  AND  PROPERTIES  OF  SOILS 

carefully  controlled  conditions,  he  found  that  unless  excessive 
amounts  of  manure  were  added  no  appreciable  effects  were 
observed.  Such  results  indicate  that  the  heat  of  decay  and 
fermentation  has  little  practical  effect  in  modifying  the  tem- 
perature of  field  soils.  Without  doubt  there  are  certain  local- 
ized influences,  but  how  important  they  may  be  is  beyond 
our  present  knowledge.  As  far  as  heat  relations  are  con- 
cerned, it  seems  that  organic  matter  exerts  its  greatest  effects 
through  a  darkening  of  the  color  and  an  increase  in  the  mois- 
ture capacity  of  the  soil. 

126.  Loss  of  heat — conduction,  radiation,  and  evapora- 
tion.— Although  small  amounts  of  heat  may  be  carried  from 
the  soil  by  percolating  water,  the  only  important  loss  is  into 
the  atmosphere  above.  This  loss  occurs  in  three  ways,  con- 
duction, radiation,  and  evaporation.  The  loss  due  to  evapora- 
tion is  easily  the  least  important  of  the  three.  Conduction 
and  radiation  have  much  to  do  with  climatic  control,  since 
the  atmosphere  receives  its  energy  in  large  degree  from  the 
earth  rather  than  directly  from  the  sun.  Conduction  from 
soil  to  air  and  vice  versa  can  be  modified  but  to  a  slight  extent 
by  man,  a  fortunate  provision  of  nature. 

Terrestrial  bodies  are  continually  radiating  energy  waves 
into  the  atmosphere,  the  change  of  temperature  depending  on 
whether  the  receipt  of  such  oscillations  exceeds  or  falls  short 
of  the  loss.  In  the  case  of  the  soil,  there  is  a  very  great  dis- 
sipation of  energy  in  this  way,  radiation  with  conduction 
being  important  climatic  controls.  The  rapid  changes  in  air 
temperature  are  often  directly  due  to  these  phenomena. 

These  energy  waves  of  terrestrial  origin  are  very  long,1 

being  within   the   infra-red   group   and   consequently   make 

no  impression  on  the  eye.     They  are  often  spoken  of  as  the 

dark  rays.     Their  energy  capacity  is  higher  than  that  of 

shorter  oscillations.     The  trapping  of  heat  in  a  greenhouse 

1  Terrestrial  bodies  at  ordinary  temperatures  give  out  waves  varying 
in  length  from  .000270  to  .001500  cm.  The  warmer  the  body,  the  shorter 
the  wave  length. 


SOIL  HEAT  241 

is  partially  due  to  the  tendency  of  the  objects  within  the  house 
to  give  off  these  long  rays,  which  do  not  pass  through  the 
glass  with  the  facility  possessed  by  the  shorter  vibrations  by 
means  of  which  a  large  proportion  of  the  energy  was  intro- 
duced. 

The  texture,  structure,  and  color  of  the  soil  have  little  in- 
fluence on  radiation.  Moisture  tends  to  hasten  it  a  trifle, 
since  water  is  a  better  radiator  than  soil.  Mulches,  as  they 
are  loose  and  dry,  may  check  radiation  slightly.  Artificial 
coverings,  shelters,  and  clouds  seem  to  exert  the  greatest  effect. 
It  is  often  feasible  to  protect  plants  from  frost  by  interfering 
with  radiation  and  conduction.  Clouds  by  shutting  in  heat, 
may  in  some  cases  prevent  a  frost  that  would  otherwise  occur, 
due  to  the  rapid  cooling.  Snow  likewise  has  a  protecting 
effect  and  may  often  prevent  the  soil  underneath  from  freez- 
ing. While  man  may  influence  radiation  locally,  it  is  evident 
that  the  total  energy  loss  can  be  checked  but  little. 

The  effect  of  evaporation  on  the  temperature  of  the  soil  is 
especially  noticeable  because  of  its  rapid  action.  This  vapor- 
ization of  water  is  caused  by  an  increased  molecular  activity 
and  requires  the  expenditure  of  a  certain  amount  of  heat,1 
which  results  in  a  cooling  effect  on  the  water  remaining  and 
consequently  on  the  soil  and  air  with  which  it  is  in  contact. 
It  requires  267.9  kilogram  calories  to  evaporate  one  pound 
of  water  at  50°  F.  This  is  sufficient  to  lower  the  temperature 
of  a  cubic  foot  of  saturated  clay  about  20°  F.,  providing  that 
all  of  the  energy  of  evaporation  comes  from  the  soil  and  its 
water. 

The  low  temperature  of  a  wet  soil  is  due  partially  to  evapo- 
ration and  partially  to  high  specific  heat.  King  2  found  during 

1  It  requires  536.6  gram-calories  to  evaporate  one  gram  of  water  at 
100 °C,  while  596.7  calories  are  necessary  if  evaporation  takes  place  at 
0°C.     The  calories  (C)  required  to  vaporize  one  gram  of  water  at  any 
temperature  (t)  may  be  calculated  by  the  formula: 
C  =  596.73  —  .601  t 

aKing,  F.  H.,  Physics  of  Agriculture,  p.   20;    Madison,  Wis.,   1910. 


242        NATURE  AND  PROPERTIES  OF  SOILS 

April  that  an  undrained  soil  in  Wisconsin  ranged  from  2.5°P. 
to  12.5°  F.  lower  than  one  of  the  same  type  well  drained. 
Parks  l  reports  data  of  the  same  order  from  England.  Drained 
and  undrained  soil  held  in  trays  at  Urbana,  Illinois,2  showed 
maximum  differences  of  13.7°  F.,  9.0°  F.,  and  6.2°  F.  at 
depths  of  1,  2,  and  4  inches,  respectively.  The  differences 
were  greatest  in  the  day.  Wollny  considers  that  the  depres- 
sion of  temperature  due  to  evaporation  is  roughly  propor- 
tioned to  the  moisture  present.  Texture,  structure,  and  or- 
ganic matter  influence  the  cooling  action  of  evaporation,  since 
they  exert  such  a  marked  effect  on  water  capacity  and  capil- 
lary movement.  The  practical  importance  of  evaporation 
study  lies  in  the  fact  that  it  can  be  controlled  to  such  a 
marked  extent  in  the  field.  Such  is  not  true  of  radiation  and 
conduction.  Windbreaks  and  shelters  have  been  shown  by 
King 3  to  reduce  evaporation  over  short  distances  as  much  as 
25  per  cent.  This  means  a  conservation  of  soil  energy  for 
the  time  being.  Thorough  under-drainage  not  only  checks 
evaporation  losses  but  lowers  the  specific  heat  of  the  soil, 
retards  its  radiation  and  facilitates  convection.  This  means 
a  faster  warming  up,  especially  of  the  root  zone.  Optimum 
moisture  encourages  optimum  heat  conditions  as  well  as  other 
favorable  phenomena.  Drainage,  tillage,  and  organic  matter 
are  the  dominant  factors  in  this  moisture  control. 

127.  Soil  temperature  and  its  variations. — The  tempera- 
ture of  the  soil  at  any  time  depends  on  the  ratio  of  the  energy 
absorbed  and  the  heat  being  lost.  The  constant  change  in 
this  coordination  is  reflected  in  the  seasonal,  monthly,  and 
daily  soil  temperatures.    The  following  data  4  are  representa- 

1  Parks,  J.,  On  the  Influence  of  Water  on  the  Temperature  of  Soils; 
Jour.  Roy.  Agr.  Soc.  Eng.,  Vol.  5,  pp.  119-146,  1845. 

"Mosier,  J.  G.,  and  Gustafson,  A.  F.,  Soil  Physics  and  Management; 
p.  302;  Philadelphia,  1917. 

8  King,  F.  H.,  The  Soil,  p.  189 ;  New  York,  1906. 

*Swezey,  G.  D.,  Soil  Temperatures  of  Lincoln,  Nebraska;  Nebr.  Agr. 
Exp.  Sta.,  16th  Ann.  Rep.,  pp.  95-102,  1903. 


SOIL  HEAT 


243 


tive  of  soil  temperatures  in  temperate  climates  with  moderate 
rainfall : 

Table  LI 

AVERAGE   TEMPERATURE   READINGS   TAKEN  AT  LINCOLN, 
NEBRASKA,  1890-1902.      DEGREES  FAHRENHEIT 


l 

3 

6 

12 

24 

36 

Season 

Air 

Inch 

Inches 

Inches 

Inches 

Inches 

Inches 

Deep 

Deep 

Deep 

Deep 

Deep 

Deep 

Summer. . . 

25.9 

28.8 

28.8 

29.5 

32.2 

36.3 

39.1 

Autumn .  . . 

49.9 

54.8 

53.6 

51.6 

48.5 

45.7 

44.3 

Spring.  .  .  . 

73.8 

83.0 

80.9 

79.1 

73.8 

69.0 

66.2 

Winter.  . .  . 

53.9 

56.4 

57.6 

57.1 

57.5 

59.3 

60.3 

It  is  apparent  that  the  seasonal  variations  of  temperature 
are  considerable  even  at  the  lower  depths.  The  surface  layers 
vary  more  or  less  in  accord  with  the  air  temperature  and, 
therefore,  exhibit  a  greater  fluctuation  than  the  subsoil.  In 
general,  the  surface  soil  is  warmer  in  spring  and  summer  than 
the  lower  layers  but  cooler  in  fall  and  winter.  The  soil,  on 
the  average,  is  warmer  than  the  air  in  winter.  This  occurs 
because  the  air  responds  more  quickly  to  a  change  in  solar 
insolation  than  the  soil. 

The  curves  showing  the  monthly  march  of  soil  temperature 
at  Lincoln,  Nebraska  (Fig.  45),  reveal  the  lag  of  the  tempera- 
ture change  in  the  subsoil  due  to  slow  heat  penetration.  It  is 
also  noticeable  that  the  monthly  range  in  temperature  change 
in  the  surface  soil  is  higher  than  that  of  the  air.  The  abso- 
lute range  is,  of  course,  greater  for  the  air.  It  must  be  kept 
in  mind  that  changes  in  soil  temperature  are  gradual,  while 
the  air  may  vary  many  degrees  in  an  hour. 

The  daily  and  hourly  temperature  of  the  air  and  soil  in 
the  temperate  zone  may  show  considerable  agreement  or 
marked  divergence  according  to  whether  the  weather  control 
is  cyclonic  or  solar.  With  solar  control  and  a  clear  sky  the 
air  temperature  rises  from  morning  to  a  maximum  at  about 


244        NATURE  AND  PROPERTIES  OF  SOILS 

two  o'clock.  It  then  falls  rapidly.  The  soil,  however,  does 
not  reach  its  maximum  temperature  until  later  in  the  after- 
noon, due  to  the  usual  soil  lag.  This  retardation  is  greater 
and  the  temperature  change  less  as  the  depth  increases.1  The 
substratum  of  a  soil  shows  little  daily,  or  even  monthly,  varia- 
tion and  is  affected,  if  at  all,  by  seasonal  changes  only.    The 


JAN.     FBSR.    MXR.     APR.     MAY    JUNE    JULY     AUG.     SEPT.    OCT.      NO*      DEC 


Fig.  45. — Curves  showing  the  average  monthly  temperature  readings  at 
various  soil  depths.     Average  of  twelve  years,  Lincoln,  Nebraska. 


curves  in  Fig.  46,  comparing  soil  and  air  temperatures  at 
Munich2  on  a  bright  day  in  May,  substantiates  some  of  the 
statements  above : 

128.  Control  of  soil  temperature. — The  most  important 
factor  in  the  control  of  soil  heat  is  obviously  moisture.  Good 

1  The  following  laws  hold  in  a  general  way : 

1.  The  lag  of  the  temperature  wave  is  proportional  to  the  depth. 

2.  The  diurnal  amplitude  of  the  temperature  oscillation  decreases  in 
geometric  progression  as  the  depth  increases  in  arithmetic  progression. 
If  the  temperature  variation  at  the  surface  was  24°F  and  at  6  inches 
deep  12°F,  according  to  this  law  the  diurnal  variation  at  12  inches 
would  be  6°F  and  at  18  inches  3°F. 

aWollny,  E.,  Untersuchungen  iiber  den  Einfluss  der  Pflanzendeclce  und 
der  Beschattung  auf  die  Physikalischen  Eigenschaften  des  Boden; 
Forsch.  a.  d.  Gebiete  d.  Agr.-Physik.,  Band  VI,  Seite  197-256,  1885. 


SOIL  HEAT 


245 


drainage,  and  a  proper  structural  development,  sufficient  or- 
ganic matter  and  deep  and  careful  plowing,  favor  optimum 
moisture  conditions.  Such  moisture  regulation  means  a  low- 
ered specific  heat,  rapid  conductivity,  and  good  convection. 
The  increase  of  soil  organic  matter  may  act  directly  in  heat 
control  by  darkening  the  color  and  thus  increasing  absorp- 


50 


& 

A 

]s 

+~ 

\1 

V 

s 

5 

f& 

I/ 

7 

\ 

s 

/ 

\ 

\ 

T/ME 

/A/  HK 

s. 

M 


JO 


A/ 


Fig.  46. — Curves  showing  the  hourly  temperature  of  a  bare  soil  at  a 
depth  of  four  inches  and  of  the  air  just  above  the  soil  in  Ger- 
many, May  26.     (Data  from  Wollny.) 

tion.    A  soil-mulch,  being  dry,  not  only  may  check  evapora- 
tion but  at  the  same  time  may  lower  radiation. 

Any  method  of  handling  the  land  which  tends  to  benefit  its 
physical  condition,  better  its  tilth  and  control  its  moisture, 
tends  at  the  same  time  towards  a  proper  heat  control.  The 
whole  question  may  be  summarized  by  saying  that,  if  a  farmer 
adopts  a  proper  system  of  moisture  control  and  at  the  same 
time  employs  methods  that  continually  encourage  a  better 


246        NATURE  AND  PROPERTIES  OF  SOILS 

physical  condition  of  the  soil,  the  problem  of  the  control  of 
soil  heat  will  be  solved  automatically.  The  farmer  will  then 
have  brought  about  the  best  conditions  for  heat  absorption 
and  will  have  facilitated  conduction  and  convection,  retarding 
at  the  same  time  losses  by  evaporation  and  radiation. 


CHAPTER  XII 
SOIL  AIR 

The  soil  is  a  porous  mass  of  material  of  which  only  about 
one-half  is  solid  matter.  The  pore  space  that  results  is  occu- 
pied by  water  and  by  air  in  a  constantly  varying  proportion. 
When  a  soil  is  in  good  condition  for  crop  growth,  the  air 
space  rarely  makes  up  more  than  from  20  to  25  per  cent,  of 
its  volume.  The  texture  of  the  soil  and  the  amount  of  mois- 
ture are  obviously  the  main  controls.  The  individual  air 
spaces  of  the  soil  are  more  or  less  continuous  and  seem  to 
maintain  a  fairly  complete  communication  between  the  vari- 
ous horizons.  The  better  the  granulation  of  the  soil  and 
the  greater  the  number  of  cracks  and  burrows,  the  easier  and 
quicker  is  this  communication.  The  air  of  the  soil  is  either 
directly  in  contact  with  the  roots  and  the  soil  bacteria  or 
separated  from  them  by  only  a  thin  layer  of  moisture  or  col- 
loidal material. 

The  air  of  the  soil  is  not  merely  a  continuation  of  the  atmo- 
spheric air  into  the  interstitial  spaces.  As  it  is  enclosed  by  the 
soil  complexes  and  by  the  soil-moisture  movement  does  not 
take  place  readily.  Hence  it  is  greatly  influenced  by  its  local 
surroundings.  This  leads  to  important  differences  between 
the  atmospheric  air  and  the  soil  air,  the  character  of  the  latter 
depending  on  a  variety  of  conditions  in  which  the  physical, 
chemical  and  biological  properties  of  the  soil  play  a  large 
part. 

129.  Composition  of  soil  air. — The  air  of  the  soil  differs 
from  that  of  the  outside  atmosphere  in  that  it  contains  more 
water-vapor,  a  much  larger  proportion  of  carbon  dioxide,  a 

247 


248        NATURE  AND  PROPERTIES  OF  SOILS 

correspondingly  smaller  amount  of  oxygen,  and  slightly  larger 
quantities  of  other  gases,  including  ammonia,  methane,  hydro- 
gen sulfide,  and  the  like,  formed  by  the  decomposition  of 
organic  matter.  The  percentage  of  nitrogen  is  practically  the 
same  in  all  cases.  The  following  average  data  quoted  from 
three  different  sources  show  the  comparative  compositions 
as  far  as  the  carbon  dioxide,  oxygen,  and  nitrogen  are  con- 
cerned. All  other  gases  are  included  with  the  nitrogen  fig- 
ures.1 

Table  LII 

AVERAGE   COMPOSITION    OF    SOIL   AIR   AND    ATMOSPHERIC    AIR 


Location 

Percentage  by  Volume 

C02 

o3 

N3 

Soil  Air 

Germany  2 

Iowa  3 

.20 
.20 
.25 

.03 

20.60 
20.40 
20.65 

20.97 

79.20 
79.40 

England4 

Atmospheric  Air 
England  4 

79.20 
79.0 

Russell  and  Appleyard,5  in  their  study  of  the  soil  atmo- 
sphere, found  that  there  are  really  two  types  of  soil  air.  The 
first  one  occupies  the  portion  of  the  pore  space  not  taken 


1  Atmosphere  air  carries  about  .93  per  cent,  of  argon,  with  very  small 
amounts  of  other  inert  gases  such  as  krypton,  xenon,  helium  and  neon. 
These  gases  are  of  course  present  in  the  soil. 

aLau,  E.,  Beitrdge  zur  Kenntnis  der  Zusammensetzung  der  im  Acker- 
boden  befindlichen  Luft;  Inaug.  Diss.,  Rostock,  1906. 

3  Jodidi,  S.  L.,  and  Wells,  A.  A.,  Influence  of  Various  Factors  on 
Decomposition  of  Soil  Organic  Matter;  la.  Agr.  Exp.  Sta.,  Ees.  Bui. 
No.  3,  Oct.  1911. 

4  Russell,  E.  J.,  and  Appleyard,  A.,  The  Atmosphere  of  the  Soil:  Its 
Composition  and  the  Causes  of  Variation;  Jour.  Agr.  Sci.,  Vol.  VII, 
Part  1,  pp.  1-48,  1915. 

5 Russell,  E.  J.,  and  Appleyard,  A.,  The  Atmosphere  of  the  Soil:  Its 
Composition  and  the  Causes  of  Variation;  Jour.  Agr.  Sci,  Vol  VII, 
Part  1,  pp.  1-48,  1915. 


SOIL  AIR  249 

up  by  water,  is  free  to  move  from  place  to  place  and  is  satu- 
rated or  nearly  saturated  with  water-vapor.  It  is  the  soil 
atmosphere  most  commonly  referred  to  and  its  composition 
is  set  forth  in  the  above  tabulation.  After  this  air  was  drawn 
off  Russell  and  Appleyard  found  that  still  more  air  could 
be  removed  by  applying  suction.  This  air  at  first  carried 
considerable  oxygen  but  by  continuing  the  suction  almost  pure 
carbon  dioxide  was  obtained.  The  amount  of  gas  removed 
by  lowering  the  pressure  varied  directly  with  the  moisture 
content  of  the  soil  and  consequently  it  may  be  considered  as 
air  largely  absorbed  by  the  moisture  of  the  soil  complexes. 

Two  types  of  atmosphere,  therefore,  exist  in  the  soil.  One, 
the  ordinary  soil  air,  is  comparatively  rich  in  oxygen.  The 
other,  absorbed  by  the  soil  moisture,  is  very  low  in  oxygen 
but  very  high  in  carbon  dioxide.  Obviously  they  insensibly 
merge.  The  biological  significance  of  these  atmospheric  types 
is  very  important.  Their  simultaneous  presence  admits  of 
both  aerobic  and  anaerobic  biological  activity.  For  example, 
rapid  nitrate  formation  might  be  progressing  but  no  accumu- 
lation would  be  evident,  due  to  just  as  rapid  a  synthetic  activ- 
ity of  the  anaerobic  forms.1 

It  must  not  be  assumed  from  the  data  above  quoted  that 
the  composition  of  the  soil  air  is  at  all  constant  or  that  it  is 
approximately  the  same  in  every  soil.  The  soil  is  dynamic 
in  nearly  every  phase  and  is  nowhere  more  changeable  than 
in  its  atmospheric  composition.  This  variability  will  of  course 
be  more  marked  and  more  important  in  the  air  which  occupies 
the  interstitial  spaces,  although  the  absorbed  air  will  show 
some  fluctuation.  The  compositions  of  the  air  of  several  soils, 
as  determined  by  Boussingault  and  Lewy2  are  quoted  in  the 
following  table : 

1  Gainey,  P.  L.,  Beal  and  Apparent  Nitrifying  Power  of  Soils;  Science, 
N.  S.,  Vol.  39,  pp.  35-37,  1914. 

Doryland,  C.  J.  T.,  Influence  of  Energy  Material  upon  the  Eelation  of 
Soil  Microorganisms  to  Soluble  Plant  Food;  N.  Dak.  Agr.  Exp.  Sta., 
Bui.  116,  pp.  318-399,  1916. 

2  Johnson,  S.  W.,  How  Crops  Feed,  p.  219;  New  York,  1891. 


250        NATURE  AND  PROPERTIES  OF  SOILS 


Table  LIII 


Character  of  Soil 

Percentage  by  Volume 

CO, 

oa 

N, 

Sandy  subsoil  of  forest 

Loamy  subsoil  of  forest 

Surface  soil  of  forest 

Clay  soil 

.24 
.79 
.87 
.66 
.74 
1.54 
3.64 

19.66 
19.61 
19.99 
19.02 
18.80 
16.45 

79.55 
79.52 
79.35 

Soil  one  year  after  manuring 

Soil  freshly  manured 

Vegetable  mold  compost 

80.24 
79.66 
79.91 

The  differences  in  the  composition  of  the  atmosphere  of 
different  soils  and  the  variability  noticeable  within  the  same 
soil  are  due  primarily  to  two  factors:  (1)  the  production  of 
carbon  dioxide,  and  (2)  oxidation.  These  will  be  discussed 
in  the  above  mentioned  order. 

130.  The  carbon  dioxide  of  the  soil  air. — The  presence 
of  carbon  dioxide  in  soils  may  be  due  in  small  part  to  in- 
filtration from  the  atmospheric  air,  there  being  a  tendency 
for  the  carbon  dioxide,  which  is  heavier  than  nitrogen  and 
oxygen,  to  settle  out.  It  may  also  have  a  purely  chemical 
origin.  The  latter  source  is  much  more  probable.  The  ab- 
sorption of  the  bases  of  carbonates  or  bicarbonates  would 
obviously  release  carbon  dioxide.  This  probably  does  not  take 
place,  however,  to  any  great  extent  in  a  natural  soil.  When 
ground  limestone  is  added,  such  a  reaction  does  occur.1  Car- 
bon dioxide  in  appreciable  amounts  might  for  a  short  time 
thus  be  liberated  through  chemical  reaction.     The  addition 


1MaeIntire,  W.  H.,  The  Carbonation  of  Burnt  Lime  in  Soils;  Soil 
Sci.,  Vol.  VII,  No.  5,  pp.  325-446,  1919.  See  also,  The  Non-existence 
of  Magnesium  Carbonate  in  Humid  Soils;  Tenn.  Agr.  Exp.  Sta.,  Bui. 
107,  1914.  8  ■ 


SOIL  AIR 


251 


of  lime  has  been  shown  by  several  investigators  to  increase 
the  carbon  dioxide  production.1 

There  is  now  no  doubt  that  biological  activities  are  largely 


4.5 

4.0 

5-5 
5.0 
ft 

gz-o 

U 

gi.5 

1.0 


"7i 

—A — ! — a? 

X  [ft 

In 


JUNE 


JULY 


AUGU5T 


SEPT. 


Fig.  47. — Diagram  showing  the  amount  of  carbon  dioxide  in  air  from 
Volusia  silt  loam  limed  and  unlimed  and  cropped  to  oats. 


responsible  for  the  occurrence  of  the  large  quantity  of  carbon 
dioxide  in  the  soil  air.    There  are  two  distinct  processes  in- 

1Bizzell,  J.  A.,  and  Lyon,  T.  L.,  The  Effect  of  Certain  Factors  on 
the  Carbon  Dioxide  Content  of  Soil  Air;  Amer.  Soc.  Agron.,  Vol.  10, 
No.  3,  pp.  97-112;  Mar.  1918. 

Potter,  E.  S.,  and  Snyder,  R.  S.,  Carbon  Dioxide  Production  in  Soils 
and  Carbon  and  Nitrogen  Changes  in  Soils  Variously  Treated;  la.  Agr. 
Exp.  Sta.,  Ees.  Bui.  39;  Feb.  1916. 

Plummer,  J.  K.,  Some  Effects  of  Oxygen  and  Carbon  Dioxide  on  Nitri- 
fication and  Ammonification  in  Soils;  Cornell  Agr.  Exp.  Sta.,  Bui.  384; 
Dec.   1916. 


252        NATURE  AND  PROPERTIES  OF  SOILS 

volved:  (1)  the  physiological  action  of  bacteria  by  which 
they  absorb  oxygen  and  give  off  carbon  dioxide,  and  (2)  the 
excretion  of  carbon  dioxide  by  roots.    (See  Fig.  47.) 

Recent  work1  has  clearly  shown  that  higher  plants,  espe- 
cially during  their  most  rapid  growth,  markedly  increase  the 
amount  of  carbon  dioxide  gas  in  the  soil.  Stoklasa2  concluded 
that  the  microorganisms  in  an  acre  of  soil  to  a  depth  of  four 
feet  may  produce  between  sixty-five  and  seventy  pounds  of 
carbon  dioxide  a  day  for  two  hundred  days  in  the  year,  and 
that  during  the  growing  period  the  roots  of  oats  or  wheat 
would  give  off  nearly  as  much  more.  Turpin3  finds  that  the 
crop  often  produces,  during  its  period  of  active  growth,  many 
times  as  much  carbon  dioxide  as  is  produced  by  soil  organ- 
isms. He  minimizes  the  influence  of  the  decaying  root  par- 
ticles of  the  crop  occupying  the  soil  on  the  carbon  dioxide 
content  of  the  soil  air. 

In  any  particular  soil,  the  two  major  controls  of  carbon 
dioxide  production  seem  to  be  temperature  and  rainfall.4  The 
former  apparently  is  dominant  in  a  temperate  humid  region 
from  November  to  May.  During  the  remainder  of  the  year, 
the  moisture  content  of  the  soil  and  the  amount  of  rainfall 
are  the  direct  controls.     Bacterial  numbers  and  nitrate  ac- 

1  Stoklasa,  J.,  and  Ernest,  A.,  Beitrdge  zur  Losung  der  Frage  der 
Chemischen  Natur  des  WurzelseJcretes ;  Jahr.  Wiss.  Bot.,  Bd.  46,  Seite 
55-102,  1909. 

Aberson,  J.  H.,  Ein  Beitrag  zur  Kenntnis  der  Natur  der  Wurzelaus- 
scheidunger;  Jahr.  Wiss.  Bot.,  Bd.  47,  Seite  41-56,  1910. 

Bussell,  E.  J.,  and  Appleyard,  A.,  The  Influence  of  Soil  Conditions 
on  the  Decomposition  of  Organic  Matter  in  the  Soil;  Jour.  Agr.  Sei., 
Vol.  VII,  Part  3,  pp.  385-417,  1917. 

Bizzell,  J.  A.,  and  Lyon,  T.  L.,  The  Effect  of  Certain  Factors  on 
the  Carbon  Dioxide  Content  of  Soil  Air;.  Amer.  Soe.  Agron.,  Vol.  10, 
No.  3,  pp.  97-112,  Mar.  1918. 

2  Stoklasa,  J.,  Methoden  zur  Bestimmung  der  Atmungsintensitat  der 
Bakterien  im  Boden.  Zeit,  f.  d.  Landw.  Versuchswesen  in  Oesterreich, 
Band  14,  Seite  1243-79,  1911. 

3  Turpin,  H.  W.,  The  Carbon  Dioxide  of  the  Soil  Air;  Cornell  Agr. 
Exp.  Sta.,  Memoir  32,  April  1920. 

*  Bussell,  E.  J.,  and  Appleyard,  A.,  The  Atmosphere  of  the  Soil:  Its 
Composition  and  the  Causes  of  Variation;  Jour.  Agr.  Sci.,  Vol.  VII, 
Part  1,  pp.  1-48,  1915. 


SOIL  AIR 


253 


cumulation  seem  to  fluctuate  with  the  carbon  dioxide,  while 
the  oxygen  curve  is  almost  the  exact  reciprocal.  Other  in- 
fluences of  a  minor  nature  enter  in,  such  as  the  character  of 
the  crop  growing  on  the  soil,  heavy  rainfall,  oxygen  dissolved 
in  the  rain,  and  rapid  changes  of  temperature.  (See  Fig. 
48.) 

While  plowing,  application  of  lime,  drainage,  and  other 
practices  have  a  great  influence  on  the  proportion  of  oxygen 


FE8R.     MARCH    APRIL. 


MAY 


JUNE        JULY 


AUG. 


Fig.  48. — Carbon  dioxide  in  air  from  Dunkirk  clay  loam  bare  and  from 
the  same  soil  cropped  to  oats,  1918.     (After  Turpin.) 


and  carbon  dioxide  in  the  soil  air,  the  addition  of  organic 
matter  seems  to  have  the  most  profound  effect.  At  the 
Rothamsted  Experiment  Station,1  the  carbon  dioxide  content 
of  the  air  from  two  soils  was  studied.  One  soil  (Broadbalk 
field)  had  been  manured  for  a  number  of  years  while  the  other 
(Hoos)  had  not  received  such  a  treatment: 

1Eussell,  E.  J.,  and  Appleyard,  A.,  The  Atmosphere  of  the  Soil:  Its 
Composition  and  the  Causes  of  Variation;  Jour.  Agr.  ScL,  Vol.  VII, 
Part  1,  p.  25,  1915. 


254        NATURE  AND  PROPERTIES  OF  SOILS 

Table  LIV 

EFFECT  OF  FARM  MANURE  ON  THE  CARBON  DIOXIDE  CONTENT  OF 
SOIL  AIR.      ROTHAMSTED,  ENGLAND 


Treatment 

Percentage  of  C02  by  Volume 

May  15 

May  25 

June  10 

June  12 

July  7 

July  27 

Manured  soil 

Unmanured  soil. . . 

.22 
.10 

.32 

.07 

.17 

.08 

.36 
.07 

.36 

.08 

.35 

.09 

Although  the  formation  of  carbon  dioxide  in  the  soil  is  in- 
fluenced to  a  marked  degree  by  the  decomposition  of  organic 
matter,  the  effect  is  by  no  means  proportional  to  the  quantity 
of  organic  matter  present.  The  rate  of  decomposition  varies 
greatly,  and  where  this  is  depressed,  as  sometimes  occurs  in 
muck  or  forest  soils,  the  content  of  carbon  dioxide  is  relatively 
low.  A  high  percentage  of  organic  matter  is  in  itself  likely 
to  prevent  a  proportional  formation  of  carbon  dioxide,  since 
the  accumulation  of  the  gas  may  inhibit  further  activity  of 
the  decomposing  organisms. 

131.  Oxidation  and  its  effect  on  the  composition  of  the 
soil  air. — Oxidative  processes  in  the  soil  are  of  two  general 
types,  those  due  to  chemical  reactions  alone  and  those  due 
to  biochemical  transformations.  The  purely  chemical  oxida- 
tion may  be  illustrated  best  by  recalling  the  processes  of  soil 
formation.1  Here  it  was  noted  that  certain  minerals,  espe- 
cially those  carrying  iron,  were  susceptible  to  the  influence  of 
oxygen.  The  following  reactions  show  how  olivine  may  as- 
sume water  and  then  produce  ferric  oxide  through  oxidation : 
3MgFeSi04  +  2H20  =  H4Mg3Si209  +  Si02  +  3FeO 
4FeO  +  02  =  2Fe203 

This  is  illustrative  of  the  complex  reactions  which  are  con- 
tinually taking  place  and  which  tend  materially  to  decrease 
the  oxygen  of  the  soil  air. 

1  See  Chapter  II,  par.  16,  of  this  text. 


SOIL  AIR  255 

Biochemical  oxidation,  however,  is  usually  rapid  and  is  a 
much  more  important  factor  in  the  oxygen  control  of  the  air. 
Not  only  do  all  bacteria  require  oxygen  for  their  growth,  but 
they  are  continually  producing  compounds  that  require  oxy- 
gen in  their  molecules.  Carbon  dioxide  is  an  oxidation 
product.  Its  formation  reduces  the  oxygen  of  the  air  and  its 
presence  causes  a  dilution.  Sulfofication  and  nitrification  are 
well  known  examples.  The  reactions  for  the  process  of  nitri- 
fication illustrate  in  addition  the  production  of  carbon  dioxide 
by  chemical  means: 

2NH3  +  302  =  2HN02  +  2H20 

2HN02  +  CaC03  =  Ca(N02)2  +  H20  +  C02 

Ca(N02)2  +  02  =  Ca(N03)2 

132.     Function  of  the  carbon  dioxide  of  the  soil. — The 

solvent  action  of  carbon  dioxide  is  probably  one  of  its  most 
important  functions  in  the  soil.  Constant  biological  activities, 
combined  with  the  seasonal  cropping  influences,  maintain  this 
solvent  and  keep  it  continually  in  contact  with  the  solution 
surfaces  of  the  soil.  Although  a  very  weak  acid  when  dis- 
solved in  water,  its  rapid  formation  and  continuous  action  is 
productive  of  marked  effects. 

The  availability  of  almost  all  of  the  plant  nutrients  is  due 
either  directly  or  indirectly  to  the  action  of  carbon  dioxide. 
Its  influence  on  the  potash  of  orthoclase,  the  phosphoric  acid 
of  tri-ealcium  phosphate  and  the  calcium  of  calcium  carbonate 
are  well  known  examples: 

2KAlSi308  +  2H20  +  C02  =  H4Al2Si209  +  4Si02  +  K2C03 

Ca3(POJ2  +  2H20  +  2C02  =  CaH4(P04)2  +  2CaC03 

CaC03  +  H20  +  C02  =  CaH2(C03)2 

Stocklasa1  has  correlated    the    carbon    dioxide  production 

1Stoklasa,  J.,  Methoden  zur  Bestimmung  der  Atmungsintensitat  der 
Bakterien  im  Boden;  Zeit.  f.  d.  Landw.  Versuchswesen  in  Oesterreich, 
Band  14,  Seite  1243-79,  1911. 


256         NATURE  AND  PROPERTIES  OF  SOILS 

with  the  quantity  of  phosphates  found  in  the  drainage  water 
from  certain  soils.    Some  of  his  results  are  given  in  Table  LV : 

Table  LV 


Soil 


Loam 

Clay....... 

Lime  soil.  . . 
Organic  soil 


Eelative  Produc- 
tion of  G02 
(milligrams  to  a 
pound  of  soil  in  24 

HOURS) 

11 

7 

16 
25 


Stoklasa  considers  that  the  production  of  carbon  dioxide 
is  a  measure  of  the  intensity  of  bacterial  action  in  the  soil, 
and  that  in  consequence  of  this  activity  the  phosphorus  is 
rendered  soluble. 

As  far  as  biological  activity  is  concerned,  carbon  dioxide 
seems  to  be  a  factor  only  insofar  as  it  dilutes  the  oxygen.1 
This  seems  to  be  especially  true  of  those  bacterial  processes 
involved  in  the  formation  of  nitrates.  When  it  exists  to  the 
exclusion  of  the  oxygen,  it  produces  anaerobic  conditions  but 
in  this  respect  it  functions  in  exactly  the  same  way  as  does 
nitrogen  or  any  other  inert  gas.  Physiologically  it  seems  to 
have  no  detrimental  effects.  Carbon  dioxide  increases  so 
markedly  with  an  increase  in  nitrate  production  that  its 
presence  can  not  be  depressing.2 

133.  Importance  of  oxygen  in  the  soil  air. — Oxygen  is 
the  all-important  gas  of  the  soil  air.    Without  it  no  weather- 

1Plummer,  J.  K.,  Some  Effects  of  Oxygen  and  Carbon  Dioxide  on 
Nitrification  and  Ammonification  in  Soils;  Cornell  Agr.  Exp.  Sta.,  Bui. 
384,  Dec.  1916. 

Also,  Owen,  W.  L.,  Effect  of  Carbonates  upon  Nitrification ;  Ga.  Agr. 
Exp.  Sta.,  Bui.  81,  1908. 

aNeller,  J.  E.,  Studies  in  the  Correlation  Between  the  Production 
of  Carbon  Dioxide  and  the  Accumulation  of  Ammonia  by  Soil  Organ- 
isms; Soil  Set,  Vol.  V,  pp.  225-241,  1918. 

Stoklasa,  Julius,  Methoden  zur  biochemischen  Untersuchung  des 
Bodens;  Handb.  Biochem.  Arbeitsmeth.,  Bd.  5,  S.  843-910,  1912. 


SOIL  AIR  257 

ing  would  occur,  no  minerals  would  break  down,  and  no  solu- 
tion would  be  possible.  Oxidation  must  go  on  rapidly  and 
continuously  in  the  normal  soil,  not  only  for  chemical  but  for 
biological  reasons  as  well.  By  it  the  organic  matter  that 
would  soon  accumulate  to  the  exclusion  of  higher  plant  life  is 
disposed  of,  and  its  nutrient  materials  are  brought  into  a 
condition  in  which  they  may  be  absorbed  by  roots.  The 
presence  of  oxygen  is  essential  either  directly  or  indirectly 
to  the  organisms  that  facilitate  decomposition.  Through  such 
a  process,  roots  of  past  crops,- as  well  as  other  organic  matter 
that  has  been  plowed  under,  are  rapidly  changed  in  the  soil. 
The  processes  of  decay  give  rise  to  products,  chiefly  carbon 
dioxide,  that  are  solvents  of  mineral  matter,  and  leave  the 
nitrogen  and  ash  constituents  more  or  less  available  for  plant 
use. 

Oxygen  is  also  necessary  for  the  germination  of  seeds  and 
the  growth  of  roots.  These  phenomena,  although  not  involv- 
ing the  removal  of  large  quantities  of  oxygen,  are  entirely  de- 
pendent on  its  presence  in  considerable  amounts. 

134.  Volume  of  the  soil  air. — The  amount  of  air  in  soils 
is  determined  by  their  physical  properties,  the  variability  in 
any  particular  soil  being  due  to  certain  changes  to  which  such 
a  soil  is  normally  subject  from  time  to  time.  The  factors 
that  influence  the  volume  of  air  in  soil  are:  (1)  texture;  (2) 
structure;  (3)  organic  matter;  and  (4)  moisture  content. 

It  is  a  well  recognized  fact  that  the  finer  the  texture,  the 

better  the  granulation  and  the  larger  the  amount  of  organic 

matter,  the  greater  is  the  amount  of  pore  space.    Since  about 

the  same  proportion  of  the  pore  space  is  filled  with  water  in 

every  soil  when  it  is  in  optimum  condition  for  crop  growth, 

it  is  obvious  that  with  finer  texture,  better  granulation  and 

increased  organic  matter,  there  will  be  a  greater  amount  of 

air  present. 

Russell,  E.  J.,  and  Appleyard,  A.,  The  Influence  of  Soil  Conditions 
on  the  Decomposition  of  Organic  Matter  in  the  Soil;  Jour.  Agr.  Sci., 
Vol.  VIII,  Part  3,  pp.  385-417,  1917. 


258         NATURE  AND  PROPERTIES  OF  SOILS 

It  must  also  follow  that  the  larger  the  proportion  of  the 
interstitial  space  filled  with  water,  the  smaller  will  be  the 
quantity  of  air  contained.  This  does  not  mean  that  the  soil 
with  the  higher  percentage  of  water  will  contain  the  least  air. 
The  percentage  pore  space,  which  is  determined  by  the  tex- 
ture, structure,  and  organic  matter  is  a  consideration  also. 
These  three  factors,  together  with  moisture  content,  are  in- 
volved in  the  following  formula  for  calculating  air  space: 

%  Air  Space  =  %  Pore  Space  —  (%H20  X  Vol.  Wt.) 

If  one  soil,  containing  30  per  cent,  of  water,  has  a  pore 
space  of  50  per  cent,  and  a  volume  weight  of  1.3,  its  air  space 
would  be  11  per  cent,  of  the  total  soil  volume.  Another  soil 
with  20  per  cent,  of  moisture,  a  pore  space  of  40  per  cent, 
and  a  volume  weight  of  1.6  would,  on  the  other  hand,  con- 
tain only  8  per  cent,  of  air.  The  above  formula,  however, 
is  irremediably  inaccurate  in  two  respects.  It  does  not  allow 
for  the  air  dissolved  in  the  soil-moisture  nor  does  it  compen- 
sate for  the  influence  of  the  gelatinous  colloidal  material  that 
exists  in  the  interstices  especially  of  a  heavy  soil. 

135.  Movement  of  soil  air. — There  seems  to  be  a  slow 
but  constant  movement  of  air  through  the  interstitial  spaces 
of  a  normal  soil  in  an  attempt  to  create  a  homogeneous  com- 
position within  the  soil  as  well  as  to  establish  equilibrium  with 
the  atmospheric  air.  The  major  controls  of  such  movement 
are  (1)  moisture  and  (2)  temperature  changes.  The  minor 
influences  are  (1)  diffusion  and  (2)  fluctuations  in  atmo- 
spheric pressure. 

As  water,  when  present  in  a  soil,  occupies  certain  of  the 
interstitial  spaces,  it  decreases  the  air  space  when  it  enters 
the  soil  and  increases  it  when  it  leaves.  The  downward  move- 
ment of  rain-water  produces  a  movement  of  soil  air  by  forcing 
it  out  through  the  drainage  channels  below,  while  at  the  same 
time  a  fresh  supply  of  air  is  drawn  in  behind  the  wave  of 
saturation  as  the  water  passes  down  from  the  surface.     The 


SOIL  AIR 


259 


movement  thus  occasioned  extends  to  a  depth  where  the  soil 
becomes  permanently  saturated  with  water.  Twenty-five  per 
cent,  of  the  air  in  a  soil  may  be  driven  out  by  normal  change 
in  moisture  content.  Capillary  movement,  whether  it  be  pro- 
duced by  evaporation,  plant  action  or  other  normal  forces, 
likewise  produces  movement  of  the  soil  air.  In  fact,  every 
readjustment  of  soil-moisture,  however  slight,  will  produce 
a  corresponding  adjustment  of  the  air  films. 

It  is  generally  considered  that  the  effect  of  normal  tempera- 
ture change  on  the  contraction  or  expansion  of  the  soil  air  is 
so  slight  as  to  produce  but  little  movement.  Ramann  says,1 
"  Since  the  coefficient  of  expansion  of  gas  is  only  1/273  to  a 
degree  Centigrade  and  since  the  temperature  fluctuations  to 
the  depths  of  from  four  to  eight  inches  are  small,  the  diurnal 
exchange  of  gas  is  consequently  slight. ' '  Bouyoucos,2  by  rais- 
ing the  temperature  of  both  dry  and  moist  soil  held  in  a 
properly  controlled  apparatus,  was  able  to  measure  the 
amount  of  air  actually  expelled.  He  found  in  every  case  that 
the  gases  driven  off  markedly  exceeded  the  theoretical 
amounts. 

Table  LVI 

EFFECT  OF  TEMPERATURE  ON  THE  AMOUNT  OF  AIR  EXPELLED  FROM 

MOIST   SOILS 


Soil 

Per 

Cent 
Mois- 
ture 

Per 

Cent 
Po- 
rosity 

Cubic  Centimeters  of  Air  Expelled 
From  One-half  Cubic  Foot  of  Soil 

Theo- 
retical 
In- 

0-10°C 

~289~ 
326 
363 
466 

10°-20°C 

20°-30°C 

30°-40°C 

for 
Each 
10°C 

Sandy  loam 
Silt   loam.  . 

Clay 

Peat 

11.0 
18.6 
25.3 
92.0 

48.2 
47.0 
50.3 
38.6 

354 
335 

382 
512 

382 
428 
428 
559 

419 
465 
503 
657 

250 
244 
261 
200 

1Eamann,  E.,  BodenTcunde,  Seite  386;  Berlin,  1905. 

2  Bouyoucos,  G.  J.,  Effect  of  Temperature  on  Some  of  the  Most 
Important  Physical  Processes  in  Soils;  Mich.  Agr.  Exp.  Sta.,  Tech.  Bui. 
22,  pp.  50-62,  1915. 


260         NATURE  AND  PROPERTIES  OF  SOILS 

Not  only  are  the  amounts  of  air  expelled  larger  than  the 
theoretical  figures,  but  the  differences  rise  with  the  tempera- 
ture. With  a  change  of  40°  C.  it  is  to  be  expected  that  the 
actual  gas  expelled  will  exceed  the  theoretical  from  1.2  to  2.7 
times,  depending  on  the  soil  and  its  condition.  This  apparent 
discrepancy  is  due  to  the  expansion  of  the  aqueous  vapor  in 
the  soil  air  and  to  the  liberation  of  absorbed  gases  with  a  rise 
in  temperature. 

Diurnal  fluctuations  in  temperature  often  rise  as  high  as 
15°  C.  for  the  upper  six  inches  of  soil  in  the  summer  months.1 
When  it  is  remembered  that  monthly  and  seasonal  differences 
are  even  greater  than  the  diurnal  and  that  this  respiring  effect 
continues  day  after  day,  the  importance  of  temperature  in 
relation  to  air  movement  cannot  be  minimized.  If  a  six-inch 
layer  of  soil  is  raised  from  5°  C.  to  20°  C.  in  temperature, 
about  10  per  cent,  of  its  atmosphere  will  be  expelled,  pro- 
viding the  actual  expansion  is  twice  the  theoretical. 

The  wide  difference  in  the  compositions  of  soil  and  atmo- 
spheric gases  give  rise  to  diffusion  movements,  especially  of 
the  oxygen  and  carbon  dioxide.  This  tendency  towards  equi- 
librium is  also  important  in  the  readjustments  within  the  soil. 
As  oxidation  and  carbon  dioxide  production  do  not  occur 
equally  in  all  parts  of  the  soil,  diffussion  movements  might 
easily  be  induced.  The  readjustments  and  equalizations  be- 
tween the  soil  air  proper  and  that  absorbed  by  the  soil-mois- 
ture are  probably  largely  diffusive.  Although  diffusion  phe- 
nomena are  slow,  Buckingham2  considers  them  quite  impor- 
tant. 

Waves  of  high  or  low  atmospheric  pressure,  frequently  in- 
volving a  change  of  0.5  inch  on  the  mercury  gauge,  are  con- 
stantly following  each  other  eastward  across  the  continent. 
Low  pressure  allows  the  soil  air  to  expand  and  issue  from 

*See  Swezey,  G.  D.,  Soil  Temperature  at  Lincoln,  Nebraska;  Nebr. 
Agr.  Exp.  Sta.,  16th  Ann.  Rep.,  pp.  95-102,  1903. 

2  Buckingham,  E.,  Contributions  to  Our  Knowledge  of  Aeration  of 
Soils;  U.  S.  Dept.  Agr.,  Bur.  Soils,  Bui.  25,  1904. 


SOIL  AIR  261 

the  soil,  while  a  high  pressure  following  causes  the  outside 
air  to  enter.  An  appreciable,  but  not  important,  movement 
of  soil  air  is  produced  in  this  way.  Gusts  of  wind,  by  affect- 
ing the  air  pressure,  would  function  in  the  same  way  but 
presumably  would  influence  only  the  superficial  air  spaces. 

136.  Practical  modification  of  soil  air. — The  ordinary 
operations  of  tillage  greatly  influence  the  ventilation  of  the 
soil.  When  a  soil  is  plowed,  the  bottom  of  the  furrow  is  ex- 
posed directly  to  the  air,  and,  by  the  separation  of  adhering 
particles  and  aggregates  of  particles,  air  is  brought  into  con- 
tact with  portions  that  previously  have  been  shut  off  from 
atmospheric  influence.  It  is  partly  because  of  its  effect  on 
soil  ventilation  that  plowing  is  beneficial.  The  necessity  for 
its  practice  is  obviously  greater  in  a  humid  region  and  on  a 
heavy  soil  than  in  a  region  of  light  rainfall  and  on  a  light 
soil.  The  practice  of  listing  corn  in  semi-arid  regions,  by 
which  the  soil  is  sometimes  left  unplowed  for  a  number  of 
years,  would  fail  utterly  on  the  heavy  soils  of  a  humid  region. 

Subsoiling,  by  loosening  the  subsoil,  increases  the  ventila- 
tion at  the  lower  depths.  Rolling  and  subsurface  packing 
both  diminish  the  volume  and  the  movement  of  air.  Their 
essential  difference  is  in  their  effect  on  moisture  rather  than 
on  air.  Harrowing  and  cultivation  have  the  opposite  effect, 
and  both  may  under  certain  conditions  increase  the  produc- 
tion of  nitrates  in  the  soil  by  promoting  aeration. 

Farm  manures,  lime,  and  other  amendments  that  improve 
the  structure  of  the  soil  have  for  that  reason  a  beneficial 
action  on  soil  aeration.  By  their  effect  on  the  physical  con- 
dition of  the  soil,  they  increase  its  permeability,  and  by  stim- 
ulating oxidation  and  carbon  dioxide  production  they  induce 
diffusion. 

Under-drainage,  by  lowering  the  water-table  and  removing 
the  soil-water  from  the  larger  capillary  spaces,  markedly  in- 
fluences the  aeration  of  the  soil  and  thus  profoundly  modifies 
the  chemical  and  biological  activities  therein.     There  is  a 


262        NATURE  AND  PROPERTIES  OF  SOILS 

very  considerable  movement  of  air  in  and  out  of  tile  drains, 
which  cannot  fail  to  influence  the  aeration  of  the  soil  above. 
The  influence  of  irrigation  on  the  soil  is  much  like  that  of 
rainfall.  The  alternate  filling  and  emptying  of  the  interstitial 
spaces  with  water  causes  a  very  considerable  change  of  air. 

The  roots  of  plants  left  in  a  soil  after  the  crop  has  been 
harvested  decay  and  leave  channels  in  the  soil  through  which 
air  penetrates.  Below  the  furrow  slice,  where  the  soil  is  not 
stirred  and  where  it  is  usually  more  dense  than  at  the  surface, 
this  affords  an  important  means  of  aeration.  The  absorption 
of  moisture  from  the  soil  by  roots  also  causes  the  air  to  pene- 
trate, in  order  to  replace  the  water  withdrawn. 

137.  Resume. — The  air  of  the  soil  differs  from  the  atmos- 
pheric air  in  being  relatively  lower  in  oxygen  and  compara- 
tively very  much  higher  in  carbon  dioxide.  It  is  generally 
saturated  with  water- vapor.  The  percentage  of  nitrogen  and 
other  gases  is  about  the  same  as  in  the  atmosphere.  The 
major  portion  of  the  soil  atmosphere  exists  in  the  larger  inter- 
stices. Its  movement  in  most  cases  is  due  to  moisture  and 
temperature  changes,  although  diffusion  and  fluctuations  in 
barometric  pressure  are  of  some  importance.  A  minor  portion 
of  the  soil  air  is  dissolved  in  the  soil-water,  the  absorptive 
influences  of  the  soil  complexes  probably  playing  a  part  also. 
Carbon  dioxide  is  the  predominating  gas  in  the  minor  por- 
tion, which  maintains  an  equilibrium  with  the  more  active 
soil  air  largely  by  diffusion. 

While  the  amount  of  air  in  the  soil  varies  with  the  texture, 
structure,  and  organic  matter,  the  moisture  content  seems  to 
be  the  dominant  factor  with  volume  as  well  as  with  movement. 
Although  plowing,  tillage,  and  manuring  profoundly  influence 
the  soil  air  and  its  relationships  to  normal  chemical  and  bio- 
logical reactions,  natural  forces  and  processes,  once  the  crop 
is  on  the  soil,  seem  to  control  aeration. 


CHAPTER  XIII 
THE  ABSORPTIVE  PROPERTIES  OF  SOILS 1 

It  has  been  known  from  very  early  times  that  soils  were 
able  to  take  up  and  tenaciously  hold  such  materials  as  salts 
and  dyes.  Aristotle,  for  example,  noticed  that  sea  water  was 
purified  when  passed  through  sand.  This  capacity  of  soil 
to  absorb  and  fix,  more  or  less  completely,  materials  added 
to  it  is  called  adsorption.  The  earliest  quantitative  experi- 
ments were  made  by  H.  S.  Thompson  in  England.  He  found 
that  the  soil  was  able  to  absorb  considerable  quantities  of 
ammonia  from  ammonium  sulfate,  the  acid  radical  being 
liberated.  The  importance  of  absorption  phenomena  has  since 
attracted  much  attention,  both  from  the  practical  and  the 
theoretical  standpoint.2 

138.  Types  of  absorption. — Two  general  types  of  absorp- 
tion are  usually  recognized,  physical3  and  chemical.  In  the 
former  case  the  absorbed  material  is  supposed  to  be  concen- 
trated on  the  surfaces  of  the  absorbing  substance,  no  chemical 
reaction  taking  place.  The  absorptive  capacity  of  charcoal 
and  cotton  for  dyes  is  a  good  example  of  such  a  phenomenon. 
In  many  cases,  however,  absorption  is  due  to  chemical  reac- 

1  The  literature  on  absorption  by  soils  is  so  complicated  and  contra- 
dictory that  only  those  concepts  which  are  more  or  less  definitely  estab- 
lished and  which  have  a  practical  bearing  on  soil  management  will  be 
considered. 

2  A  good  review  of  literature  will  be  found  as  follows : 

Patten,  H.  E.,  and  Waggaman,  W.  H.,  Absorption  by  Soils;  U.  S. 
Dept.  Agr.,  Bur.  Soils,  Bui.  52,  1908. 

Prescott,  J.  A.,  The  Phenomenon  of  Absorption  in  its  Relation  to 
Soils;  Jour.  Agr.  Sci.,  Vol.  VIII,  No.  1,  pp.  111-130,  Sept.,  1916. 

'Physical  absorption  is  sometimes  spoken  of  as  adsorption.  The  ten- 
dency at  present  is  toward  the  elimination  of  this  term. 

263 


264        NATURE  AND  PROPERTIES  OF  SOILS 

tion.  The  tenacity  with  which  soils  absorb  and  hold  phos- 
phoric acid  is  probably  due  to  the  change  that  the  soluble 
form  undergoes  almost  immediately  in  the  soil,1  producing 
the  sparingly  soluble  tri-calcium  phosphate  (Ca3(P04)2)  or 
the  practically  insoluble  iron  and  aluminum  phosphates 
(FeP04  and  A1POJ. 

While  it  is  generally  considered  that  most  of  the  material 
absorbed  by  soil,  whether  the  action  is  chemical  or  physical, 
is  concentrated  at  the  surfaces  of  the  solid  material,  there  is 
some  evidence  that  part  of  it  penetrates,  forming  a  solid  solu- 
tion. For  example,  the  longer  a  gas  is  held  at  high  pressure 
within  an  absorbing  material,  the  less  will  be  released  when 
the  pressure  is  lowered.  Again,  while  most  absorption  is 
almost  instantaneous,  the  final  equilibrium  is  very  slow.  Such 
phenomena  have  given  rise  to  a  theory  of  molecular  invasion. 

In  the  soil  it  is  impossible  to  know  whether  the  absorption 
of  any  material  has  been  purely  physical,  purely  chemical,  or 
due  to  both  actions.  In  all  probability  both  types  of  fixation 
occur.  When  a  potassium  compound  is  added  to  a  soil,  the 
potassium  is  taken  up  very  readily.  The  fixation  at  first  is 
probably  physical.  This  type  of  absorption  generates  chem- 
ical reactions  catalytically  and  the  remainder,  and  possibly 
the  greater  proportion  of  the  fixation,  is  probably  chemical 
in  nature. 

139.  Causes  of  absorption. — Way2  was  the  first  to  ad- 
vance any  definite  explanation  of  absorption.  After  study- 
ing the  absorptive  capacity  of  double  silicates  of  sodium  and 
aluminum,  he  decided  that  the  phenomenon  was  purely  chem- 

1CaH4(P04)2  +  2CaH2(C08)2  =  Ca,(P04)2  +  4H20  +  40O2 
Soluble  Insoluble 

2  Way,  J.  T.,  On  the  Tower  of  Soils  to  Absorb  Manure;  Jour.  Koy. 
Agr.  Soc,  England,  Vol.  11,  pp.  313-379,  1850.  Also,  On  the  Power  of 
Soils  to  Absorb  Manure;  Jour.  Boy.  Agr.  Soc,  England,  Vol.  13,  pp. 
123-143,  1852.  Also,  On  the  Influence  of  Lime  on  the  "Absorptive 
Properties"  of  Soils;  Jour.  Eoy.  Agr.  Soc,  England,  Vol.  15,  pp.  491- 
515,  1854. 


THE  ABSORPTIVE  PROPERTIES  OF  SOILS    265 

ical.  Warington  1  also  believed  in  the  chemical  hypothesis. 
Liebig,  however,  regarded  absorption  as  largely  physical.  Van 
Bammelen  2  was  the  first  to  direct  attention  to  the  importance 
of  both  organic  and  inorganic  colloidal  matter  to  absorption 
phenomena.  This  type  of  explanation  seems  the  most  plau- 
sible in  light  of  present  knowledge  of  the  colloidal  state  of  cer- 
tain soil  constituents  and  from  the  fact  that  a  soil  very  often 
does  not  remove  different  bases  in  chemically  equivalent 
amounts.3  The  fact  that  a  soil  apparently  saturated  with  one 
base  is  able  to  absorb  quantities  of  another  is  additional  argu- 
ment against  a  purely  chemical  explanation. 

In  the  soil,  especially  if  it  is  of  a  clayey  nature,  there  always 
exist  certain  quantities  of  hydrated  aluminum  silicates  of 
indefinite  chemical  constitution.  They  are  generally  colloidal 
in  nature.4  Such  materials,  as  well  as  those  of  an  organic 
character,  possess  high  absorptive  capacities,  not  only  be- 
cause of  their  tremendous  surface  exposures  but  also  because 
of  their  tendency  to  react  quickly  and  easily  with  substances 
in  the  soil  solution.    According  to  Van  Bemmelen,  who  made 

1  Warington,  K.,  On  the  Part  Taken  by  Oxide  of  Iron  and  Alumina 
in  Absorptive  Action  of  Soils;  Jour.  Chem.  Soc,  (London),  Vol.  6, 
pp.  1-19,  1868. 

2  Van  Bemmelen,  J.  M.,  Die  Absorptionsverbindungen  und  das  Absorp- 
tionsvermbgen  der  Acker erde;  Landw.  Vers.  Stat.,  Band  35,  Seite  75, 
1888.     Also,  Die  Absorption,  Dresden,  1910. 

3  The  uncertainty  regarding  the  real  explanation  of  absorption  is 
shown  by  the  controversy  of  Weigner,  who  holds  to  the  colloidal  theory, 
with  Gans,  who  believes  the  phenomenon  is  chemical. 

Weigner,  G.,  The  Chemical  or  Physical  Nature  of  Colloidal  Aluminum 
Silicates  Containing  Water;  Centrbl.  f.  Min.  u.  Palaontol.,  No.  9,  pp. 
262-272,  1914. 

Gans,  R.,  Concerning  the  Chemical  or  Physical  Nature  of  Colloidal 
Water-containing  Aluminum  Silicates;  Centrbl.  f.  Min.  u.  Palaontol., 
No.  22,  pp.  699-712;  No.  23,  pp.  728-741,  1914. 

4  The  absorptive  capacity  of  the  soil  is  often  ascribed  to  zeolites. 
The  presence  of  zeolites  in  the  soil,  however,  is  extremely  improbable. 
Water  and  the  absence  of  oxidizing  agents  are  essential  for  their  for- 
mation. They  are  products  of  hydrometamorphism  and  not  of  weather- 
ing. It  seems  probable  that  the  processes  of  weathering  are  not  only 
opposed  to  zeolite  formation  but  would  destroy  those  already  present. 

Merrill,  G.  P.,  Weathering  of  Micaceous  Gneiss;  Bui.  Geol.  Soc.  Amer., 
Vol.  8,  pp.  162-166,  1879. 


266        NATURE  AND  PROPERTIES  OF  SOILS 

a  very  exhaustive  study  of  the  subject,  the  following  colloidal 
materials  may  function  in  the  soil : 

1.  Partially  decayed  remains  of  plant  and  animal  tissue. 

2.  Colloidal  iron,  aluminum,  and  silica. 

3.  Colloidal  silicates  formed  through  weathering. 

Van  Bemmelen  also  credits  crystalline  silicates  with  some 
absorptive  power,  but  he  does  not  consider  such  action  par- 
ticularly important. 

The  combinations  produced  by  absorption  are  often  weak, 
it  being  possible  to  leach  out  the  substances  held  in  the  water 
of  the  colloidal  gels.  The  following  example  of  one  kind  of 
absorption  is  given  by  Van  Bemmelen  x  and  shows  how  com- 
plex the  phenomenon  may  become:  ten  grams  of  a  hydrogel 
having  the  composition  Si02.4.2H20,  shaken  with  100  cubic 
centimenter  solution  of  20  molecular  equivalent  KC1,  absorbed 
0.8  to  1.1  molecular  equivalent  of  the  dissolved  substance. 
The  absorption  in  this  case  was  as  if  the  solution  had  been 
diluted  with  4.2  to  5.8  centimeters  of  water.  As  the  amount 
of  gel  water  in  10  grams  of  hydrogel  of  Si02  is  about  5  cubic 
centimeters,  the  assumption  may  be  made  that  the  dissolved 
substance  is  taken  up  in  equal  concentration  by  the  gel  water. 
Ten  grams  of  hydrogel  of  Si02  shaken  with  100  cubic  centi- 
meter solution  of  50  molecular  equivalent  KC1 — that  is,  two 
and  a  half  times  the  concentration  of  the  former  solution — 
absorbs  two  and  a  half  times  as  much,  or  2.1  to  2.5  molecular 
equivalent.  This  applies  also  to  concentrations  five  times 
stronger  than  the  first  mentioned  above,  but  beyond  that  the 
relation  is  not  so  simple.  It  serves,  however,  to  illustrate 
the  manner  in  which  the  absorption  takes  place  from  dilute 
solutions. 

140.     The  absorptive  capacity  of  soils.2 — The  absorptive 

1Van  Bemmelen,  J.  M.,  Die  Absorptionsverbindungen  und  das  Ab- 
sorptionsvermogen  der  Acker erde;  Landw.  Vers.  Stat.,  Band  35,  Seite 
75,  1888. 

2 A  few  important  citations  are  as  follows: 

Peters,  E.,  tfeber  die  Absorption  von  Kali  durch  AcTcererde;  Landw. 
Ver.  Stat.,  Bd.  2,  Seite  113-151,  1860. 


THE  ABSORPTIVE  PROPERTIES  OF  SOILS    267 

capacity  of  any  particular  soil  for  gases,  water,  or  salts  in 
solution,  under  any  particular  condition,  depends  on  the  tex- 
ture of  the  soil  and  on  the  time  during  which  the  action  is 
allowed  to  continue.  The  absorptive  power  of  a  soil  may  be 
determined  by  percolating  a  solution  of  known  strength 
through  a  column  of  the  soil  or  by  shaking  the  sample  with 
a  definite  amount  of  the  solution.  The  following  data  from 
Parker1  were  obtained  by  shaking  a  35-gram  portion  of  soil 
for  two  days  with  a  solution  carrying  the  equivalent  of  about 
6.5  grams  of  KC1 : 

Table  LVII 

EFFECT  OF  TEXTURE  ON  THE  ABSORPTION  OF  POTASSIUM. 


Soil  Type 

Potassium  Absorbed 
E&pressed  as  Milligrams 
of  KC1. 

Cecil  clay 

325 

Decatur  clay  loam 

Carrington  loam . 

240 
225 

Norfolk  sandy  loam 

148 

Sullivan,  E.  C,  The  Interaction  Between  Minerals  and  Water  Solu- 
tions; U.  S.  Geol.  Survey,  Bui.  312,  1907. 

Morse,  F.  W.,  and  Curry,  B.  E.,  Beactions  Between  Manurial  Salts  and 
Clay,  Mucks  and  Soils;  N.  H.  Agr.  Exp.  Sta.,  29th  Ann.  Eep.,  pp.  271- 
293,  1908. 

.  Demolon,  A.,  and  Bronet,  G.,  Sur  la  Penetration  des  Engrais  Solubles 
dans  les  Sols;  Ann.  Agron.,  Tome  28,  pp.  401-418,  1911. 

Bogue,  E.  H.,  Absorption  of  Potassium  and  Phosphorus  Ions  by 
Typical  Soils;  Jour.  Phys.  Chem.,  Vol.  19,  No.  8,  pp.  665-695,  1915. 

McCall,  A.  G.,  Hildebrandt,  F.  M.,  and  Johnston,  E.  S.,  The  Ab- 
sorption of  Potassium  by  the  Soil;  Jour.  Phys.  Chem.,  Vol.  20,  No.  1, 
pp.  51-63,  1916. 

McBeth,  J.  G.,  Fixation  of  Ammonia  in  Soils;  Jour.  Agr.  Kes., 
Vol.  IX,  No.  5,  pp.  141-155,  1917. 

Wyckoff,  M.  I.,  Absorption  of  Ammonium  Sulfate  by  Soils  and  Quartz 
Sand;  Soil  Sci.,  Vol.  Ill,  No.  6,  pp.  561-564,  1917. 

Kelley,  W.  P.,  and  Cummins,  A.  B.,  Chemical  Effect  of  Salts  on 
Soils;  Soil  Sci.,  Vol.  XI,  No.  2,  pp.  139-159,  Feb.,  1921. 

barker,  E.  G.,  Selective  Absorption  by  Soils;  Jour.  Agr.  Kes.,  Vol.  1, 
No.  5,  pp.  179-188,  Dec.,  1913. 


268        NATURE  AND  PROPERTIES  OF  SOILS 

It  is  noticeable  that  the  absorption  increases  with  the  fine- 
ness of  the  texture,  indicating  that  the  heavier  the  soil,  the 
greater  is  the  amount  of  material  present  that  possesses  marked 
capacity  for  fixation.  Organic  matter,  in  general,  does  not 
seem  as  efficacious  as  mineral  material  in  absorptive  reac- 
tions, especially  those  involving  salts. 


!00Op.p 

.m. 

c^ 

600 

*oo 

C}£L 

vo^H^ 

200JL- 

s££2£ 

^soi}z~~ 

200 


400 


600 


600 


1000 


??00  CC. 


Fig.  49. — Curves  showing  the  absorption  of  K  in  parts  per  million  by 
various  soils  from  a  solution  containing  200  parts  to  the  million  of 
K.    The  volume  of  the  percolate  is  used  as  the  abscissas. 

The  influence  of  time  on  absorption  is  shown  by  the  follow- 
ing data  from  Schreiner  and  Failyer.1  In  this  case  100  gram 
portions  of  soil  were  treated  with  500  c.c.  of  a  mono-calcium 
phosphate  solution  carrying  100  parts  per  million  of  P04.    The 


1  Schreiner,  O.,  and  Failyer,  G.  H.,  The  Absorption  of  Phosphates 
and  Potassium  by  Soils;  U.  S.  Dept.  Agr.,  Bur.  Soils,  Bui.  32,  p.  9, 
1906. 


THE  ABSORPTIVE  PROPERTIES  OF  SOILS    269 


parts  per  million  of  P04  absorbed  after  certain  intervals  of 
time  are  given  below.    (See  also  Fig.  49)  : x 

Table  LVIII 

EFFECT  OF  TIME  AND  TEXTURE  ON  THE  ABSORPTION  OF  P04  FROM 
A  SOLUTION  OF  CaH4(P04)2. 


«• 

Time 

PC4  Absorbed  in  Parts 
Per  Million 

Clayey 
Soil 

Fine  Sandy 
Soil 

3  minutes 

400 
410 
415 
435 
440 
445 

235 

40  minutes 

255 

1  hour 

260 

2  hours 

315 

4  hours 

335 

24  hours 

370 

It  must  not  be  inferred  that,  when  a  solution  is  brought 
in  contact  with  a  soil,  it  always  becomes  weaker  because  of 
absorption.  Negative  absorption  may  occur  in  which  the  sol- 
vent is  taken  up  more  rapidly  than  the  solute.  Concentra- 
tion is  thus  induced. 

141.  Selective  absorption.2 — The  fixation  phenomena  by 
the  soil,  whether  physical  or  chemical,  is  of  two  types:  (1)  the 
absorption  of  molecules,  the  compound  being  taken  up  un- 
changed; and  (2)  the  absorption  of  ions.     In  the  first  case, 

1  The  law  which  appears  to  govern  absorption  of  phosphates  and 
potash  by  the  soil  may  be  expressed  mathematically  as  follows: 

dy 


dv 


K  (A— Y) 


in  which  K  is  a  constant,  A  the  maximum  quantity  possible  for  the  soil 
to  absorb  and  y  the  quantity  actually  fixed  when  v,  volume  of  the 
solution,  has  percolated  through.  A  short  discussion  of  the  mathematics 
of  this  law  may  be  found  in  the  following  publication:  Schreiner,  O., 
and  Failyer,  G.  H.,  The  Absorption  of  Phosphates  and  Potassium  by 
Soils;  U.  S.  Dept.  Agr.,  Bur.  Soils,  Bui.  32,  pp.  23-24,  37-39,  1906. 

2  A  very  good  discussion  of  selective  absorption  is  found  in  the 
following:  Parker,  E.  Gr.,  Selective  Absorption  by  Soils;  Jour.  Agr. 
Kes.,  Vol.  1,  No.  5,  pp.  179-188,  1913. 


270         NATURE  AND  PROPERTIES  OF  SOILS 

if  a  residue  is  left,  it  is  unchanged  except  in  concentration. 
Such  would  be  the  case  in  the  absorption  of  certain  dyes,  of 
gases  and  of  hydroxides  of  various  kinds,  where  the  molecule 
is  fixed  intact.  This  first  form  of  absorption  is  by  no  means 
as  important  as  the  selective  absorption  of  ions. 

Certain  compounds,  called  electrolytes,1  tend  when  in  solu- 
tion to  ionize  or  split  up  into  ions.  Thus  potassium  nitrate, 
a  neutral  salt,  breaks  up  into  K+  and  NO"3  ions,  the  degree 
of  ionization  depending  on  the  concentration  of  the  solution. 
When  such  a  solution  is  brought  into  contact  with  soil,  the 
latter  usually,  but  not  always,  exerts  a  greater  affinity  for  the 
basic  ion,  leaving  an  excess  of  the  acid  radical  in  solution. 
The  water  present  furnishes  small  amounts  of  H+  and  OH' 
ions,  thereby  encouraging  the  formation  of  KOH,  which  is 
absorbed  intact,  together  with  the  K+  and  OH"  ions.  This 
action,  therefore,  leaves  the  H+  and  NO~3  ions  preponderant  in 
the  solution,  which  is  of  necessity  acid  in  reaction  due  to  the 
hydrogen  ion  concentration.  This  selective  absorption  may  be 
demonstrated  with  any  neutral  salt  and  any  neutral  absorbent, 
the  resultant  extract  always  being  acid  due  to  the  selective 
absorption  of  the  basic  ions. 

142.  Substitution  of  bases.2 — Associated  with  the  selec- 
tive absorption  of  bases  from  solution  there  is  a  liberation  of 

1  According  to  the  theory  of  the  electrolytic-dissociation  or  ioniza- 
tion, many  compounds  under  certain  conditions  break  up  into  electrically 
charged  portions  called  ions.  Ions  may  be  single  atoms  or  a  group  of 
atoms.  Many  inorganic  substances  are  almost  completely  ionized.  A 
few  organic  compounds  exhibit  marked  dissociation  but  many  are  not 
appreciably  affected. 

Water  dissociates  into  H+  and  OH-  ions  to  the  extent  of  about  .00001 
of  a  per  cent,  or  1  part  in  10,000,000.  An  acid  yields  hydrogen  ions 
and  other  ions  carrying  the  remainder  of  the  molecules.  Alkalies  give 
hydroxyl  ions  and  other  ions  consisting  of  the  remaining  portion  of  the 
molecules.  The  acidity  or  alkalinity  of  a  solution  is  determined  by  its 
hydrogen-ion  concentration. 

2 Van  Bemmelen,  J.  M.,  Das  Absorptionsvermogen  der  Ackererde; 
Landw.  Vers.  Stat.,  Band  21,  Seite  135-191,  1877. 

Sullivan,  E.  C,  The  Interaction  between  Minerals  and  Water  Solu- 
tions; U.  S.  Geol.  Survey,  Bui.  312,  1907. 

Wiegner,  G.,  Zum  Basenaustausch  in  der  Ackererde;  Jour.  Landw., 
Band  60,  Seite  111-150,  197-222.  1912. 


THE  ABSORPTIVE  PROPERTIES  OF  SOILS    271 


other  bases  from  the  soil,  which  appear  in  the  filtrate  as  ions 
and  in  combination  with  acid  radicals.  Such  phenomena  may- 
be considered  as  mere  basic  exchange,  pushed  forward  by  the 
mass  action  of  the  ion  absorbed,  and  is  called  substitution  of 
bases.  The  change  may  be  illustrated  as  follows: 
KC1  +  Xn  Silicate  ^  XnCl  +  K  Silicate 

It  is  unlikely  that  this  reaction  actually  takes  place  to  any 
extent  in  fertilizer  practice.1  It  is  more  probable  that  the 
acid  produced  by  the  selective  absorption  liberates  the  bases 
from  their  loose  union  with  the  hydrated  aluminum  silicate 
complexes. 

HC1  +  Xn  Silicates  ?±  XnCl  +  H  Silicates 

A  dilute  solution  of  potassium  chloride  filtered  through  a 
soil  will  produce  a  filtrate  containing  some  calcium,  mag- 
nesium, or  chloride  or  all  of  these  salts  and  some  potassium 
chloride.  The  more  dilute  the  solution,  the  larger  will  be  the 
proportion  retained,  but  the  less  the  total  quantity  absorbed. 
Peters  2  treated  100  grams  of  soil  with  250  cubic  centimeters 
of  a  solution  of  potassium  salts,  and  found  that  the  potassium 
of  separate  salts  was  retained  in  different  proportions,  and 
that  the  more  concentrated  solutions  lost  relatively  less  than 
the  weaker  ones,  although  more  actual  potassium  was  re- 
moved from  the  former. 

Table  LIX 


Solution 

Grams  of  K20 
Absorbed  From  a 
1/10  Normal  Solu- 
tion 

Grams  of  K20 
Absorbed  From  a 
1/20  Normal  Solu- 
tion 

KC1 

.3124 
.3362 
.5747 

.1990 

K2S04 

.2098 

K„C03 

.3134 

1  Parker,  E.  G.,  Selective  Absorption  by  Soils;  Jour.  Agr.  Kes.,  Vol.  1, 
No.  5,  p.  180,  1913. 

2  Peters,  E.,  vber  die  Absorption  von  Kali  durch  Aclcererde;  Landw. 
Vers.  Stat.,  Band  2,  Seite  113-151,  1860. 


272        NATURE  AND  PROPERTIES  OF  SOILS 

The  same  bases  are  not  always  absorbed  in  the  same  propor- 
tion by  different  soils;  one  soil  may  have  a  greater  absorp- 
tive power  for  potassium,  while  another  may  retain  relatively 
more  ammonia.  They  seem  to  be  somewhat  interchangeable, 
as  any  absorbed  base  may  be  released  by  a  number  of  others 
in  solution.  The  absorptive  power  of  a  soil  for  certain  bases 
is  reflected  in  the  composition  of  the  drainage  water  from  the 
soil.  The  latter  varies  with  the  soil,  and  a  soluble  fertilizer 
applied  to  one  soil  will  have  a  different  effect  on  the  composi- 
tion of  drainage  water  than  if  applied  to  another  soil.  This 
is  well  illustrated  from  lysimeter  experiments  by  Gerlach l 
at  Bromberg.  Several  soils  were  used,  a  portion  of  each  being 
fertilized  and  unfertilized  respectively.  The  lysimeters  were 
1.2  meters  deep  and  contained  4  cubic  meters  of  soil.  The 
drainage  water  was  collected  and  analyzed  for  four  years. 
The  first  yeai  there  was  no  crop,  the  second  year  potatoes  were 
grown,  the  third  oats,  and  the  fourth  rye.  The  following  re- 
sults were  obtained : 

Table  LX 

AVERAGE  COMPETITION   OF   DRAINAGE  WATER   IN  PARTS  PER    MIL- 
LION.     BROMBERG. 


Soil 

Treatment 

Total 

N 

NO, 

Or- 
ganic 

N 

K20 

CaO 

Moor  soil 

Fertilized 

32.7 

30.0 

2.7 

32.2 

405 

Untreated 

65.0 

60.3 

4.7 

26.2 

507 

Sand  low  in  or- 

ganic matter. . . 

Fertilized 

25.5 

25.1 

.4 

25.1 

92 

Untreated 

20.9 

20.4 

.5 

8.5 

90 

Sandy  loam  high 

m  organic 

matter 

Fertilized 

67.8 

64.6 

3.1 

70.2 

399 

Untreated 

69.5 

66.1 

3.4 

47.4 

414 

1  Gerlach,  U.,  Vber  die  durch  sickcrwasser  dem  Boden  Entzogenen 
Menge  Wasser  und  Nahrstoffe;  111.  Landw.  Zeitung,  30  Jahrgrange,  Heft 
95,  Seite  871-881,  1910. 


THE  ABSORPTIVE  PROPERTIES  OF  SOILS    273 

143.  Importance  of  absorption. — Absorption  is  impor- 
tant, not  only  because  it  allows  the  soil  to  retain  certain  nutri- 
ents against  excessive  leaching,  but  because  it  facilitates  the 
condensation  and  concentration  of  gases  within  the  soil.1  Rus- 
sell and  Appleyard 2  have  shown  that  the  inner  soil  air  is 
held  very  tightly  and  must  in  consequence  be  under  consid- 
erable pressure.  Such  gas  absorption  tends  to  force  reactions 
which  otherwise  would  be  very  slow.  A  part  of  the  catalytic 
power  of  the  soil  may  be  accounted  for  in  this  way.  Moreover, 
the  absorption  of  water  by  the  soil  is  by  no  means  unimportant. 
It  is  because  of  such  phenomena  that  the  moisture  of  the  soil 
occurs  in  various  forms  and  possesses  distinctly  different  re- 
lationships to  the  plant. 

The  selective  absorption  of  the  basic  ions  by  soils  of  every 
type  is  important  in  a  number  of  ways.  In  the  first  place, 
potassium,  calcium,  magnesium,  and  iron  function  in  the  soil 
as  bases.  Selective  absorption  tends  to  conserve  these  nutri- 
ents to  the  exclusion  of  their  acid  radicals,  which  are  readily 
lost  in  drainage.  Phosphorus,  however,  has  a  different  status, 
for  although  it  is  held  as  a  part  of  an  acid  radical  (P04),  it  is 
saved  from  leaching  by  the  insolubility  of  the  compounds 
which  tend  to  form.  In  the  second  place,  selective  absorption 
apparently  produces  residues  when  fertilizers  are  added  and 
these  residues  are  almost  always  acid.  Sodium  nitrate,  am- 
monium sulphate,  potassium  chloride,  and  potassium  sulphate 
will  leave  an  acid  residue  in  the  soil  solution  unless  influenced 
by  extraneous  factors,  such  as  the  addition  of  lime  or  the  ac- 
tion of  plants. 

1  Patten,  H.  E.,  and  Gallagher,  F.  E.,  Absorption  of  Vapors  and  Gases 
by  Soils;  U.  S.  Dept.  Agr.,  Bur.  Soils,  Bui.  51,  1908. 

McGeorge,  W.,  Absorption  of  Fertiliser  Salts  by  Hawaiian  Soils;  Haw. 
Agr.  Exp.  Sta.,  Bui.  35,  p.  32,  1914. 

Cook,  E.  C,  Factors  Affecting  the  Absorption  and  Distribution  of 
Ammonia  Applied  to  Soils;  Soil  Sci.,  Vol.  II,  No.  4,  pp.  305-344,  1916. 

aEussell,  E.  J.,  and  Appleyard,  A.,  The  Atmosphere  of  the  Soil:  Its 
Composition  and  the  Causes  of  Variation;  Jour.  Agr.  Sci.,  Vol.  VII, 
Part  1,  pp.  1-48,  1915. 


274        NATURE  AND  PROPERTIES  OF  SOILS 

The  acidity  of  soils,  which  is  a  function  not  only  of  the  soil 
solution  but  of  the  solid  portions  also,  is  frequently  attributed 
to  certain  absorptive  phenomena,  one  idea  being  that,  due 
to  physical  and  chemical  absorption  of  bases,  a  concentra- 
tion of  the  hydrogen  ion  is  produced  and  actual  acidity  re- 
sults. Basic  exchange  seems  to  liberate  iron  and  aluminum, 
the  salts  of  which  easily  hydrolize  and  yield  acid  solutions. 
If,  as  some  investigators  maintain,  the  toxic  principle  of  the 
so-called  acid  soils  is  active  aluminum,  manganese  or  similar 
elements,  absorption  may  again  be  the  activating  phenomenon, 
since  an  unsatisfied  absorptive  capacity,  especially  for  cal- 
cium and  magnesium,  seems  to  favor  the  presence  of  such 
constituents  in  the  soil  solution. 

The  absorptive  power  of  the  soil  is  a  controlling  factor  as 
far  as  the  composition  and  concentration  of  the  soil  solution 
is  concerned.  Any  study  of  the  dynamic  relationships  of  the 
water  solution  that  exists  in  the  soil  interstices  and  in  the  col- 
loidal complexes  which  coat  the  soil  particles,  must  reckon 
with  absorption  phenomena  and  all  of  the  factors  which  tend 
to  influence  them. 


CHAPTER  XIV 
THE  SOIL  SOLUTION 

The  soil  is  a  heterogeneous  mixture  of  solids,  gases,  and  a 
liquid.  The  mineral  constituents  come  from  the  debris  of 
rock,  the  organic  matter  is  derived  from  plant  and  animal 
tissue,  while  through  and  around  these  complex  materials  the 
water  and  gases  of  the  soil  circulate  in  ever-changing  propor- 
tions. Minute  organisms  are  also  present  in  great  numbers, 
aiding,  through  their  enzymic  activities,  the  intricate  trans- 
formations. As  a  result  of  the  reactory  inter-relations  of  the 
soil  components,  a  solution  is  generated  which  tends  to  come 
into  equilibrium  with  the  solids  and  gases  with  which  it  is 
in  contact.  As  it  is  from  this  source  that  plants  obtain  their 
mineral  nutrients,  the  soil  solution  and  its  control  demand 
especial  attention. 

The  fundamental  error  of  many  soil  conceptions  has  been 
to  regard  the  soil  as  a  static  system.  Chemical,  physical,  and 
biological  activities  are  admitted,  but  they  have  been  regarded 
as  of  little  importance  in  influencing  the  soil  mass  as  a  whole. 
Such  a  conception  is  in  error  as  every  constituent  of  the  soil 
is  dynamic.  The  presence  of  large  amounts  of  material  in 
a  colloidal  state  makes  the  constancy  of  any  particular  con- 
dition impossible  over  any  extended  period. 

In  studying  the  soil  solution,  especially  as  to  its  composi- 
tion and  concentration,  the  phenomenon  of  absorption  can 
not  be  ignored.  The  tendency  of  certain  portions  of  the  soil 
to  go  into  solution,  while  other  parts  are  absorbing  both  the 
solvent  and  the  solute,  must  be  reckoned  with.  Moreover, 
the  losses  of  nutrients  to  the  plant  and  through  leaching  are 

275 


276        NATURE  AND  PROPERTIES  OF  SOILS 

a  factor  to  be  considered.  Obviously  the  concentration  and 
composition  of  the  soil  solution  is  first  of  all  a  function  of 
the  absorptive  capacity  of  the  soil  complexes,  modified  by  the 
rate  of  solution  and  the  magnitude  of  crop  and  leaching 
activities. 

144.  Absorption  and  the  soil  solution. — In  a  bare  moist 
soil,  where  there  is  no  evaporation  or  leaching  to  disturb 
equilibrium  tendencies,  the  soil  presents  a  three-phase 
system.  The  phases  are:  (1)  the  solution  surfaces,1 
(2)    the    absorptive    or    colloidal    surfaces,    and    (3)     the 

SOLUTION 
SURFACES 


ABSORPTION         *-  SOIL 

COMPLEXES  -^ SOLUTION 

Fig.  50. — Diagram  showing  the  equilibrium  tendencies  that  exist  between 
the  solution  surfaces,  the  colloidal  complexes  and  the  soil  solution. 

soil  solution  itself.  When  solution  takes  place,  the  con- 
stituents so  affected  are  acquired  in  part  by  the  soil  mois- 
ture as  a  solute  and  in  part  by  the  absorptive  complexes. 
There  is  a  constant  attempt  at  equilibrium,  which  of  course 
is  never  attained  as  long  as  solution  continues.  Under  field 
conditions,  many  other  disturbing  factors  enter.  The  rate 
of  solution  may  vary,  and  the  capacity  and  character  of  the 
absorbing  colloidal  complexes  are  always  changing.  Moreover, 
the  amount  of  water  in  the  soil  is  never  constant,  due  to 
drainage  and  evaporation.     The  feeding  of  the  plant,  as  re- 

1  This  term  refers  to   the   soil  surfaces   from  which   solution   takes 
place. 


THE  SOIL  SOLUTION  277 

gards  both  water  and  nutrients,  and  losses  by  leaching,  must 
always  be  considered.  In  addition,  the  effect  of  tillage  as 
well  as  the  common  practices  of  adding  farm  manure,  plow- 
ing under  of  green-crops  and  applying  fertilizers  and  lime, 
are  constantly  effective  in  obstructing  equilibrium  adjust- 
ments.1    (See  Fig.  50.) 

The  soil  solution  is,  therefore,  markedly  dynamic  in  char- 
acter, constantly  changing  in  composition  and  concentra- 
tion. Its  important  control  is  absorption,  the  absorptive  sur- 
faces acting  as  a  depository,  in  which  active  reserve  nutrients 
are  held.  As  the  solution  is  depleted  in  any  constituent, 
quicker  adjustment  takes  place  between  the  solvent  and  the 
colloidal  complexes  than  is  possible  between  the  solution  and 
the  solution  surfaces.  Rapid  adjustments,  as  far  as  the  sup- 
ply of  nutrients  for  plants  is  concerned,  is  possible  only  be- 
cause of  the  absorptive  properties  of  the  colloidal  complexes 
of  the  soil. 

145.  Methods  of  studying  the  soil  solution. — Questions 
regarding  the  soil  solution  are  difficult  to  answer  because  no 
adequate  procedure  has  been  devised  for  extracting  a  repre- 
sentative sample  of  the  solution  as  it  existed  in  the  soil.  More- 
over, no  wholly  satisfactory  method  has  been  perfected  for 
its  measurement  in  place.  Various  extractive  methods  have 
been  tried.  Briggs  and  McLane  2  attempted  to  sample  the 
solution  by  the  use  of  a  centrifuge  developing  a  force  of  two 
or  three  thousand  times  that  of  gravitation.  When  the  soil 
contained  a  rather  large  quantity  of  capillary  water,  a  small 
amount  of  it  could  be  removed  in  this  way. 

*Bouyoucos  has  shown  that  even  under  controlled  conditions  the 
equilibrium  between  finely  ground  minerals  and  water  is  not  absolute 
or  real  due  to  the  complex  hydration  and  hydrolysis  which  continually 
occur.  Bouyoucos,  G.  J.,  Bate  and  Extent  of  Solubility  of  Minerals 
and  Bocks  under  Different  Treatments  and  Conditions ;  Mich.  Agr.  Exp. 
Sta.,  Tech.  Bui.  50,  July,  921. 

2  Briggs,  Lyman  J.,  and  McLane,  John  W.,  The  Moisture  Equivalent  of 
Soils;  U.  S.  Dept.  Agr.,  Bur.  Soils,  Bui.  45,  pp.  6-8,  1907. 


278        NATURE  AND  PROPERTIES  OF  SOILS 

Another  device,  perfected  by  Briggs  and  McCall,1  consists 
of  a  close-grained,  unglazed  porcelain  tube,  closed  at  one  end 
and  provided  at  the  other  with  a  tubulure,  by  which  it  can 
be  connected  with  an  exhausted  receiver.  This  tube  is  mois- 
tened and  buried  in  the  soil.  If  the  moisture  content  of  the 
soil  is  sufficient  to  reduce  the  pressure  of  the  capillary  water 
surface  in  the  soil  to  less  than  half  the  difference  between  the 
pressure  inside  and  outside  of  the  tube,  there  will  be  a  move- 
ment of  water  inward.  The  water  may  be  collected  and  ana- 
lyzed. 

More  recently  Van  Suchtelen  has  used  another  method  to 
obtain  the  soil  solution.2  He  replaces  the  soil-water  by  means 
of  paraffin  in  a  liquid  state,  at  the  same  time  subjecting  the 
soil  on  a  filter  to  suction.  The  displaced  water  is  considered 
to  represent  the  soil  solution.  Later  Van  Suchtelen  and  Itano 
substituted  pressure  for  suction,  modifying  the  apparatus  to 
meet  the  new  procedure.  This  apparatus  has  been  further 
perfected  by  Morgan.3  Lipman  4  has  proposed  a  method  in 
which  very  high  pressure,  a  minimum  of  53,000  pounds  to  the 
square  inch,  is  utilized  in  squeezing  out  the  soil-water.5 

All  such  methods  are  open  to  the  objection  that  the  sample 
is  not  representative.    The  soil  solution  changes  both  in  con- 

1  Briggs,  L.  J.,  and  McCall,  A.  G.,  An  Artificial  Boot  for  Inducing 
Capillary  Movement  of  Soil  Moisture;  Science,  N.  S.,  Vol.  20,  pp. 
566-569,  1904. 

3  Van  Suchtelen,  F.  H.  H.,  Methode  zur  Gewinnung  der  Natiirlichen 
Bodenlosung ;  Jour.  f.  Landw.,  Band  60,  Seite  369-370,  1912. 

3  Morgan,  J.  F.,  The  Soil  Solution  Obtained  by  the  Oil  Pressure 
Method;  Mich.  Agr.  Exp.  Sta.,  Tech.  Bui.  28,  Oct.,  1916. 

4 Lipman,  C.  B.,  A  New  Method  of  Extracting  the  Soil  Solution;  Univ. 
Cal.  Pub.,  Agr.  Sci.,  Vol.  3,  No.  7,  pp.  131-134,  1918.  Ramann,  E.,  et  ah, 
have  proposed  a  similar  method  but  with  less  pressure.  Internat.  Mit.  f . 
Bodenkunde,  Bd.  6,  Seite  27,  1916. 

For  a  good  criticism  of  this  method,  see  Northrup,  Zea,  Science, 
N.  S.,  Vol.  XLVII,  No.  1226,  p.  638,  June  1918. 

5Ischerekov  in  1907  used  ethyl  alcohol  to  displace  the  water  in  a 
soil  column  utilizing  only  the  force  of  gravity.  Parker  claims  that 
this  method  is  of  considerable  value.  He  found  that  data  so  obtained 
compared  closely  with  that  obtained  from  the  water  extract  method. 
Parker,  F.  W.,  Methods  of  Studying  the  Concentration  of  the  Soil 
Solution;  Soil  Sci.,  Vol.  XII,  No.  3,  pp.  209-232,  1921. 


THE  SOIL  SOLUTION  279 

centration  and  composition  so  readily  that  the  addition  of  ex- 
traneous material  or  the  exertion  of  unnatural  pressure  defeat 
the  object  of  the  determination.  Moreover,  the  soil  solution  is 
probably  not  homogeneous  and  unless  practically  all  of  it  is 
removed  a  sample  of  value  cannot  be  obtained.  The  signifi- 
cance of  such  a  sample,  if  it  were  attained,  is  questionable, 
as  it  is  impossible  to  know  the  proportion  of  the  soluble  nu- 
trients that  may  actually  be  appropriated  by  the  growing 
plant. 

The  method  of  obtaining  soil  extracts  has  been  used  to  a 
greater  extent  than  any  other  in  studying  the  soil  solution. 
Water  is  the  usual  solvent.  The  Bureau  of  Soils  filter  method x 
is  commonly  followed.  As  might  be  expected,  it  is  purely 
arbitrary  in  its  procedure,  the  idea  being  to  make  the  results 
comparative  rather  than  strictly  quantitative.  Soil  and  water 
in  the  proportions  of  1  to  5  are  mixed,  stirred  three  minutes 
and  allowed  to  stand  twenty  minutes.  The  supernatant  liquid 
is  then  forced  through  a  Pasteur-Chamberland  filter  and  a 
clear  extract  obtained  for  analysis. 

The  solution  obtained  is  not  representative  of  the  soil-water 
and  its  solutes.  It  is  only  an  extract  of  the  soil.  The  addi- 
tion of  a  large  amount  of  water  is  a  disturbing  factor.  The 
concentration  of  the  extract  is  also  modified  by  the  absorptive 
power  of  the  soil,  being  relatively  greater  for  a  sandy  than 
for  a  clayey  soil.  Moreover,  the  differential  influence  of  the 
solvent  comes  into  play,  for  as  soon  as  solution  begins,  the 
solvent  is  no  longer  pure  water  but  a  solution  of  constantly 
changing  efficiency.  Nevertheless,  the  work  of  Hoagland, 
Stewart  and  Burd  2  indicates  that  there  is  not  only  a  relation- 

1Schreiner,  O.,  and  Failyer,  G.  EL,  Colometric,  Turbidity  and  Titra- 
tion Methods  Used  in  Soil  Investigations ;  U.  S.  Dept.  Agr.,  Bur.  Soils, 
Bui.  31,  1906. 

3  Hoagland,  D.  E.,  The  Freezing  Point  Method  as  an  Index  of  Varia- 
tions in  the  Soil  Solution  Due  to  Season  and  Crop  Growth;  Jour.  Agr. 
Ees.,  Vol.  XII,  No.  6,  pp.  369-395,  1918. 

Stewart,  G.  E.,  Effect  of  Season  and  Crop  Growth  in  Modifying  the 
Soil  Solution;  Jour.  Agr.  Ees.,  Vol.  XII,  No.  6,  pp.  311-368,  1918. 


280        NATURE  AND  PROPERTIES  OF  SOILS 

ship  between  the  water  extract  of  a  soil  and  its  productivity, 
but  a  correlation  with  the  strength  of  the  soil  solution  as  well. 
The  extract  method  is  especially  valuable  in  studying  the 
nitrates  of  the  soil  solution.  As  nitrate  nitrogen  does  not 
suffer  as  much  absorption  as  do  the  nutrient  bases,  that  which 
appears  in  the  extract  is  a  fair  measure  of  the  strength  of  the 
soil  solution  insofar  as  this  constituent  is  concerned. 

The  only  method  for  measuring  the  concentration  of  the  soil 
solution  in  situ  is  that  of  Bouyoucos.1  This  is  known  as  the 
depression  of  the  freezing  point  method.  It  is  possible,  when 
dealing  with  a  pure  solution  of  a  known  salt,  to  calculate  its 
concentration  by  determining  how  much  the  freezing  point  is 
lowered  or  depressed  below  0°  C.  This  principle  is  applied 
to  the  soil  by  using  a  Beckman  thermometer  and  the  proper 
control  apparatus.  As  the  soil  solution  carries  a  great  num- 
ber of  different  ions  in  unknown  proportions,  it  is  impossible 
to  calculate  even  the  concentration  with  accuracy,  a  factor 
of  somewhat  doubtful  validity  being  utilized.  The  procedure 
gives  nothing  regarding  the  presence  of  specific  ions  nor  are 
its  results  uniform,  due  to  the  variable  dissociation  of  the  salts 
present.  Nevertheless  the  method  has  thrown  much  light 
on  the  many  difficult  problems  of  the  soil  and  its  solution. 

146.  Qualitative  composition  of  the  soil  solution. — Once 
the  dynamic  character  of  the  soil  solution  is  conceded,  three 
points  of  importance  immediately  demand  attention:  (1)  the 
qualitative  composition  of  the  soil  solution  and  its  concentra- 
tion in  toto,  (2)  the  quantitative  composition,  and  (3)  the 
factors  most  important  in  influencing  both  the  composition 
and  the  concentration  of  the  solution. 

It  must  be  recognized  at  the  outset  that  the  soil  solution 

Burd,  J.  S.,  Water  Extractions  of  Soils  as  Criteria  of  their  Crop 
Producing  Power;  Jour.  Agr.  Ees.,  Vol.  XII,  No.  6,  pp.  297-309,  1918. 

Hoagland,  D.  R.,  Martin,  J.  C,  and  Stewart,  G.  B.,  Relation  of  the 
Soil  Solution  to  the  Soil  Extract;  Jour.  Agr.  Res.,  Vol.  XX,  No.  5, 
pp.  381-395,  1920. 

1  Bouyoucos,  G.  J.,  Further  Studies  on  the  Freezing  Point  Lowering  of 
Soils;  Mich.  Agr.  Exp.  Sta.,  Tech.  Bui.  31,  Nov.,  1916. 


THE  SOIL  SOLUTION  281 

is  generally  dilute  except  in  arid  regions  under  conditions  of 
alkali.  The  concentration  probably  very  seldom  exceeds  30,- 
000  parts  per  million  and  is  normally  very  much  lower.  More- 
over, the  greater  proportion  of  the  solute  is  in  an  ionic  state, 
molecules  appearing  only  when  the  concentration  is  relatively 
high.  It  is  well  to  note  that  the  plant  absorbs  most  of  its 
nutrients  in  the  ionic  condition. 

From  the  knowledge  obtained  by  the  analysis  of  soil  ex- 
tracts, it  is  safe  to  assume  that  all  of  the  common  bases  and 
acid  radicals  normally  occur  in  the  soil  solution.  Thus,  K+, 
Na+,  Mg++,  Ca++,  Fe+++,  Ar++  and  NH4+  ions  may  be  expected 
as  well  as  such  ions  as  SO;,  SiOf,  CI-,  POJ,  NO;,  NO;  and 
COg.  Since  water  dissociates  slightly,  H+  and  OH-  ions  will 
also  be  present.  The  reaction  of  the  solution  will  depend  on 
its  hydrogen-ion  concentration  and  may  be  alkaline,  neutral 
or  acid  as  the  case  may  be.  Most  soil  solutions  seem  to  be 
slightly  acid,1  possibly  due  to  the  action  of  carbon  dioxide. 

Morgan  2  found  on  an  examination  of  the  solutions  obtained 
from  soils  by  the  oil  pressure  method  that,  as  the  moisture  in- 
creased, the  concentration  of  the  solution  decreased.  These 
findings  are  amply  corroborated  by  the  work  of  Bouyoucos  3 
with  the  depression  of  the  freezing  point  method.  The  latter 
presents  data  regarding  the  actual  concentrations  at  various 
moisture  contents,  which  seem  to  indicate  the  general  differ- 
ences that  may  be  expected  between  soils  of  different  types. 

1  Gillespie,  L.  J.,  The.  Reaction  of  Soil  and  Measurements  of  Hydro- 
gen-ion Concentration;  Jour.  Wash.  Acad.  Sci.,  Vol.  6,  No.  1,  pp. 
7-16,  1916. 

Sharp,  L.  T.,  and  Hoagland,  D.  E.,  Acidity  and  Adsorption  in  Soils 
as  Measured  by  the  Hydrogen  Electrode;  Jour.  Agr.  Res.,  Vol.  VII, 
No.  3,  pp.  123-145,  1916. 

Hoagland,  D.  R.,  Relation  of  the  Concentration  and  Reaction  of  the 
Nutrient  Medium  to  the  Growth  and  Absorption  of  the  Plant;  Jour1. 
Agr.  Res.,  Vol.  XVIII,  No.  2,  pp.  73-117,  1919. 

'Morgan,  J.  F.,  The  Soil  Solution  Obtained  by  the  Oil  Pressure 
Method;  Mich.  Agr.  Exp.  Sta.,  Tech.  Bui.  28,  1916. 

'Bouyoucos,  G.  J.,  Further  Studies  on  the  Freezing  Point  Lowering 
of  Soils;  Mich.  Agr.  Exp.  Sta.,  Tech.  Bui.  31,  pp.  14-15,  1916. 


282        NATURE  AND  PROPERTIES  OF  SOILS 


Table  LXI 

THE  CONCENTRATION  OF  THE  SOLUTION  OF  VARIOUS  SOILS  AS  DE- 
TERMINED BY  THE  DEPRESSION  OF  THE  FREEZING  POINT.    EX- 
PRESSED IN  PARTS  PER  MILLION  BASED  ON  DRY  SOIL. 


Soil 

Moisture 
% 

Concentra- 
tion 

P.   P.    M. 

Moisture 

% 

Concentra- 
tion 

P.  P.  M. 

Superior  clay . . . 
Miami  silt  loam . 
Carrington  loam 
Plainfield  sand. 
Peat 

18.8 
8.8 

15.2 
5.0 

61.3 

,   29,268 

19,560 

16,390 

6,342 

23,333 

39.4 
36.0 
38.5 
24.6 
208.5 

415 
707 
463 
366 
2,222 

147.  Quantitative  composition  of  the  soil  solution. — 
Data  regarding  the  relative  or  actual  quantities  of  the  nutri- 
ent elements  in  the  soil  solution  are  not  only  very  meagre  but 
unreliable.  Morgan  *  found,  on  comparing  the  solutions  ob- 
tained from  different  soils  by  the  oil  pressure  method,  that  the 
potassium  (K)  might  vary  from  4  to  180  parts  per  million 
based  on  dry  soil;  the  phosphorus  (P04)  from  .2  to  4.6,  and 
the  calcium  (Ca)  from  6  to  1000  parts  per  million.  King,2  in 
his  extensive  work  with  soil  extracts,  found  the  nitrate  nitro- 
gen (N03)  extremely  variable,  ranging  from  a  fraction  of  a 
part  per  million  to  more  than  150  parts  per  million  in  the  same 
soil  at  different  times.  A  greater  fluctuation  is  to  be  expected, 
however,  in  the  nitrate  nitrogen  than  with  the  other  elements, 
since  the  presence  of  soluble  nitrogen  in  the  soil  solution  is  due 
very  largely  to  biological  activity.  The  following  figures  from 
Morgan,  although  the  different  samples  should  not  be  com- 
pared, show  what  may  be  expected  in  general  regarding  the 
concentration  of  particular  elements  in  the  soil  solution. 

1  Morgan,  J.  F.,  The  Soil  Solution  Obtained  by  the  Oil  Pressure 
Method;  Mich.  Agr.  Exp.  Sta.,  Tech.  Bui.  28,  1916. 

2  King,  F.  H.,  Investigations  in  Soil  Management ;  U.  S.  Dept.  Agr. 
Bur.  Soils,  Bui.  26,  1905. 


THE  SOIL  SOLUTION 


283 


Table  LXII 

THE  AMOUNTS  OF  POTASSIUM,  PHOSPHORUS,  AND  CALCIUM  IN  THE 

SOLUTION    OF    VARIOUS    SOILS    AS    DETERMINED    BY    THE    OIL 

PRESSURE  METHOD.      EXPRESSED  IN  PARTS  PER  MILLION 

BASED  ON  DRY  SOIL. 


Moisture 

Per- 
centage 

Parts  Per  Million 

Soils 

K 

P04 

Ca 

NH3+NO3 

+N02 

Fine  sandy  loam. 
Medium  sandy- 
loam 

29.7 
27.2 

41.9 

37.8. 

24.5 

132.9 

7.18 

9.82 

12.44 

27.02 

11.03 

139.33 

1.54 

1.41 

1.85 
4.64 
1.13 
2.19 

9.10 

12.75 

37.12 

25.93 

10.56 

213.70 

.91 
13.56 

Clyde  fine  sandy 
loam 

3.80 

Miami  silt  loam. . 

Miami  clay 

Peat 

1.20 

i.61 

33.91 

Morgan's  data  indicate  that  the  least  variation  may  be  ex- 
pected in  the  phosphorus  (P04)  content,  which  does  not  differ 
greatly  in  different  soil  solutions  nor  does  it  vary  to  any  great 
extent  in  the  same  soil.  Potassium  (K)  and  especially  cal- 
cium (Ca)  show  considerable  fluctuation,  as  does  the  nitrate 
nitrogen  (N03),  as  has  already  been  emphasized.  The  figures 
of  Morgan  correlate  fairly  well  with  the  data  obtained  by  the 
Bureau  of  Soils x  by  means  of  centrifugal  extraction.  The 
potassium  (K)  averaged  about  28  parts  per  million  based  on 
the  solution,  the  calcium  (Ca)  32,  and  the  phosphorus  (POJ 
8  parts  per  million. 

148.  Influence  of  season  and  crop  on  the  soil  solution. — 
It  has  already  been  emphasized  that  the  concentration  and 
the  composition  of  the  soil  solution  suffer  wide  fluctuations. 
The  principal  causes  of  such  variations  are  as  interesting  as 

1  Cameron,  F.  K.,  The  Soil  Solution;  p.  40,  Easton,  Pa.,  1911. 


284        NATURE  AND  PROPERTIES  OF  SOILS 

they  are  important  since  they  have  a  bearing  not  only  on  the 
chemical  and  biological  phenomena  within  the  soil  but  also 
on  its  plant  relationships. 

The  broadest  and  most  general  factors  affecting  the  soil 
solution  are  season  and  crop.  Whether  the  soil  is  fallow  or 
covered  with  vegetation,  a  great  seasonal  influence  is  evident 
on  the  soil  and  its  solution.  Stewart,1  working  in  California 
with  extracts  from  thirteen  soils  held  in  large  containers, 
found  notable  fluctuations  of  nitrates,  calcium,  potassium,  and 
magnesium  both  in  bare  and  cropped  earth.  The  phosphates 
did  not  show  great  variation.  The  soluble  nutrients  were 
markedly  higher  in  the  bare  soils,  the  differences  between  the 
various  types  being  quite  noteworthy.  The  good  soils  seemed 
to  have  the  more  concentrated  soil  solution,  a  conclusion  al- 
ready reached  by  a  number  of  investigators.2  When  crops 
were  growing  on  these  soils,  the  concentration  of  soluble  nu- 
trients not  only  was  lower  than  with  the  fallowed  areas,  but 
it  was  about  the  same  in  every  type  of  soil.  The  inherent 
solution  capacity  of  the  different  soils  was  roughly  indicated 
by  the  crop  growth.    Hoagland's  3  study  of  the  concentration 

1  Stewart,  G.  R.,  Effect  of  Season  and  Crop  Growth  in  Modifying 
the  Soil  Solution;  Jour.  Agr.  Res.,  Vol.  XII,  No.  6,  pp.  311-368,  1918. 

'Snyder,  H.,  The  Water-Soluble  Plant  Food  of  Soils;  Science,  N.  S., 
Vol.  19,  No.  491,  pp.  834-835,  1904. 

King,  F.  H.,  Investigations  in  Soil  Management;  Madison,  Wis., 
1904. 

King,  F.  H.,  Investigations  in  Soil  Management;  U.  S.  Dept.  Agr., 
Bur.  Soils,  Bui.  26,  1905. 

Mitscherlieh,  E.  A.,  Eine  Chemische  Bodenanalyse  fur  Pflanzen- 
physiologische  Forschungen;  Landw.  Jahrb.,  Bd.  36,  Heft  2,  S.  309-369, 
1907. 

Lyon,  T.  L.,  and  Bizzell,  J.  A.,  The  Plant  as  an  Indicator  of  the 
Belative  Density  of  the  Soil  Solutions;  Proc.  Amer.  Soc.  Agron.,  Vol. 
IV,  pp.   35-49,   1912. 

Hall,  A.  D.,  Brenchley,  W.  E.,  and  Underwood,  T.  M.,  The  Soil  Solu- 
tion and  the  Mineral  Constituents  of  the  Soil;  Philosoph.  Trans.  Roy. 
Soc,  London,  Series  B,  Vol.  204,  pp.  179-200,  1913. 

Pantanelli,  E.,  Bicerche  Sulla  Concentrazione  del  Liquide  Circolante 
nei  Terreni  Libici;  Bui.  Orto  Bot.  R,,  Univ.  Napoli,  T.  4,  pp.  371-383. 

3  Hoagland,  D.  R.,  The  Freezing  Point  Method  as  an  Index  of  Varia- 
tions in  the  Soil  Solution  Due  to  Season  and  Crop  Growth;  Jour.  Agr. 
Res.,  Vol.  XII,  No.  6,  pp.  369-395,  1918. 


THE  SOIL  SOLUTION 


285 


of  the  solution  in  these  soils  through  the  growing  season  by 
the  freezing  point  method  corroborates  the  conclusions  drawn 
from  the  water  extracts.  The  investigation  also  indicates  that 
large  amounts  of  nutrients  are  made  available  by  cultivation, 
fallowing,  and  cropping  and  that,  from  the  standpoint  of  the 
soil  solution,  the  ordinary  farm  practices  are  inherently  sound. 
Hoagland  's  data  regarding  some  of  the  soils  studied  is  given 
in  Table  LXIII.  The  moisture  content  was  approximately  the 
same  for  each  soil. 

Table  LXIII 

THE  CONCENTRATION  OF  THE  SOIL  SOLUTION  IN  PARTS  PER  MIL- 
LION FROM  A  GOOD  AND  POOR  SOIL  EACH  FALLOWED  OR 
CROPPED   TO  BARLEY. 


Fertile  Soil 

Poor  Soil 

Date 

FALLOW 

CROPPED 

FALLOW 

CROPPED 

July  10 

July  24 

Aug.  21 

Oct.  23 

Dec.  18 

Feb.  12 

May  7 

2000 
1700 
1800 
4300 
3400 
4200 
6700 

1200 
500 
700 
1900 
1500 
1900 
3800 

1100 
800 
1300 
2900 
1800 
2700 
6300 

600 

200 

400 

900 

1000 

1800 

3700 

Further  investigations  of  Hoagland  with  Martin  x  indicate 
that  the  effect  of  cropping  on  the  soil  solution  persists  for 
a  considerable  period.  A  marked  relationship  was  also  noted 
between  the  soil  solution  and  the  physical  condition  of  the 
soil,  due  to  a  change  in  the  colloidal  matter  with  season.  An 
increase  in  colloidal  matter  was  noted  when  the  soil  solution 
was  depleted  of  its  solutes  by  plant  activities. 

1  Hoagland,  D.  E.,  and  Martin,  J.  C,  Effect  of  Season  and  Crop 
Growth  on  the  Physical  State  of  the  Soil;  Jour.  Agr.  Res.,  Vol.  XX, 
No.  5,  pp.  397-404,  1920. 


286 


NATURE  AND  PROPERTIES  OF  SOILS 


149.  Other  factors  influencing'  the  soil  solution. — A  num- 
ber of  other  conditions,  which  are  really  phases  of  season, 
influence  both  the  concentration  and  the  composition  of  the 
soil  solution.  Among  these  are  temperature,  leaching,  and 
the  moisture  content  of  the  soil.  As  the  soil  warms  up  in  the 
spring,  reactions  of  all  kinds  are  stimulated  and  an  increase 
in  concentration  generally  results.  If  considerable  rain-water 
enters  the  soil,  the  soil  solution  is  much  diluted.  It  is  also 
changed  in  composition,  due  to  the  equilibrium  adjustments 
that  of  necessity  occur.  The  following  data  from  Bouyoucos x 
show  the  influence  of  change  in  moisture  on  the  concentra- 
tion of  the  soil  solution : 

Table  LXIV 

CONCENTRATION  OF  THE  SOLUTION  OF  CERTAIN  SOILS  AT  VARIOUS 

MOISTURE    CONTENTS.       LOWERING     OF    THE    FREEZING 

POINT    METHOD. 


Soils 

Moisture 

% 

Concen- 
tration 

P.  P.   M. 

Moisture 

% 

Concen- 
tration 

P.  P.   M. 

Sand 

Sandy  loam. . . . 
Loam 

2.60 

8.30 

11.18 

17.40 

18.80 

3,939 
13,639 
13,780 
20,153 
28,940 

21.98 
21.53 
20.97 
34.76 
36.50 

303 
606 

848 

Silt  loam 

Clav 

1061 
•  1030 

If  the  soil  is  moistened  beyond  its  water-holding  capacity, 
it  is  obvious  that  drainage  losses  will  occur,  which  will  deplete 
the  soil  of  valuable  constituents.  Increase  of  moisture,  there- 
fore, may  modify  the  soil  solution  temporarily  or  permanently, 
according  to  conditions. 

Tillage  and  the  addition  of  various  materials  also  have  a 

1  Bouyoucos,  G.  J.,  The  Freezing  Point  Method  as  a  New  Means  of 
Measuring  the  Concentration  of  the  Soil  Solution  Directly  in  the  Soil; 
Mich.  Agr.  Exp.  Sta.,  Tech.  Bui.  24,  1915. 


THE  SOIL  SOLUTION  287 

remarkable  influence  on  the  soil  solution,  especially  increasing 
its  concentration  during  the  warmer  seasons.  Plowing  and 
cultivation  by  stimulating  biological  activity  may  enhance  ni- 
trate production  to  a  marked  degree  in  a  short  time.  Aeration 
will  often  increase  the  available  mineral  elements  by  the  en- 
couragement of  reactions  which  favor  solution.  The  addition 
of  salts  of  various  kinds  has  been  shown  by  Bouyoucos  *  to 
influence  the  soil  solution  profoundly.  The  compounds  added 
affected  different  soils  in  a  diverse  manner.  When  neutral 
salts  were  added,  the  soil  solution  was  increased  from  35  to 
100  per  cent,  of  the  added  strength  of  the  salts.  In  the  case 
of  phosphate  salts  the  increase  was  very  much  less. 

150.  The  soil  solution  and  productivity. — As  the  crop 
obtains  its  nutrients  from  the  soil  solution,  there  must  be  a 
direct  relationship  between  the  fertility  of  the  soil  and  the  con- 
centration and  composition  of  the  soil  solution.  The  data 
quoted  from  Hoagland  indicate  in  a  broad  way  that  a  fertile 
soil  is  capable  of  maintaining  a  more  concentrated  soil  solu- 
tion than  is  a  poorer  one.  The  work  of  other  investigators 
amply  corroborates  this  assumption.2  One  rather  convincing 
experiment  may  be  quoted. 

Hall,  Brenchley,  and  Underwood  3  analyzed  the  water  ex- 
tract from  certain  plats  on  the  Rothamsted  Experiment  Sta- 
tion farm,  the  fertilizer  treatment  and  the  yields  of  which 
had  been  recorded  for  a  long  term  of  years.  Complete  analyses 
of  the  soil  from  the  several  plats  were  also  made : 

1  Bouyoucos,  G.  J.,  The  Freezing  Point  Method  as  a  Means  of  Studying 
Velocity  Reactions  Between  Soils  and  Chemical  Agents  and  Behavior  of 
Equilibrium;  Mich.  Agr.  Exp.  Sta.,  Tech.  Bui.  37,  1917. 

Also,  Bate  and  Extent  of  Solubility  of  Soils  under  Different  Treat- 
ments and  Conditions;  Mich.  Agr.  Exp.  Sta.,  Tech.  Bui.  44,  1919. 

See  also,  Spurway,  C.  H.,  The  Effect  of  Fertilizer  Salt  Treatments  on 
the  Composition  of  Soil  Extracts;  Mich.  Agr.  Exp.  Sta.,  Tech.  Bui.  45, 
1919. 

2  See  citations  page  284. 

3  Hall,  A.  D.,  Brenchley,  W.  E.,  and  Underwood,  T.  M.,  The  Soil 
Solution  and  the  Mineral  Constituents  of  the  Soil;  Phil.  Trans.  Roy. 
Soc,  London,  Series  B,  Vol.  204,  pp.  179-200,  1913. 


288        NATURE  AND  PROPERTIES  OF  SOILS 


Table  LXV 

YIELDS  TO  THE  ACRE  OP   CROPS,  AND   COMPOSITION  OF   SOIL  AND 

WATER  EXTRACT  OP  SOIL.       ROTHAMSTED  EXPERIMENT 

STATION    FARM.       ENGLAND. 


Treatment 

Yield  to 
the  Acre 

Complete  Analy- 
sis 

Water  Extract 

(pounds) 

PA 

% 

K20 

% 

PA 

P.  P.  M. 

K20 

P.  P.  M. 

Untreated 

N  +  K20 

1276 
2985 
3972 
5087 
6184 

.099 
.102 
.173 
.182 
.176 

.183 
.257 
.248 
.326 
.167 

.525 

.808 
3.900 
4.025 
4.463 

3.40 
30.33 

N  +  P205 

N  +  K20  +  P205- 
Farm  manure .... 

3.88 
24.03 
26.45 

151.  Summary. — The  solution  as  it  exists  in  a  normal 
soil  is  highly  dynamic.  Its  concentration  and  composition 
are  fundamentally  governed  by  rate  of  solution,  by  absorp- 
tion, and  by  the  amounts  of  the  various  solutes  in  the  solution 
itself.  Many  factors  are  active  in  preventing  a  condition  of 
equilibrium  between  these  three  phases.  Those  of  especial 
importance  are  season  and  crop.  Temperature,  moisture  con- 
tent, and  leaching  are  subfactors  of  season.  Tillage  of  all 
kinds  and  the  addition  of  manures,  lime,  and  fertilizers  are 
practical  means  of  modifying  the  soil  solution  moie  to  ade- 
quately meet  the  needs  of  the  crop.  In  fact,  all  of  the  com- 
mon practices  so  successfully  used  in  economic  soil  manage- 
ment attain  their  end  through  a  modification  and  control  of 
the  soil  solution. 


CHAPTER  XV 

THE  REMOVAL  OF  NUTRIENTS  FROM  THE  SOIL  BY 
CROPPING  AND  LEACHING 

The  soil  solution,  because  of  its  dynamic  character,  offers 
two  sources  of  loss  for  nutrient  materials,  one  of  which  should 
be  economically  encouraged,  while  the  other  should  be  reduced 
by  suitable  control  to  as  low  a  point  as  is  consistent  with  good 
soil  management.  These  two  sources  of  exhaustion  are  (1) 
cropping  and  (2)  leaching  or  drainage.  One  is  a  legitimate 
expenditure ;  the  other  is  a  waste,  which  within  certain  limits 
in  a  humid  region  is  unavoidable.1 

152.  Intake  of  water  by  plants — osmosis. — Plants  ob- 
tain their  raw  materials  from  the  air  and  the  soil,  the  former 
furnishing  the  carbon  and  the  oxygen,  most  of  the  water  and 
the  nutrients  proper  coming  from  the  soil.  Although  many 
constituents,  some  necessary  and  some  incidental,  pass  into 
the  plant  from  the  soil,  for  convenience  of  discussion  two 
groups  may  be  established:  (1)  water,  and  (2)  nutrients  prop- 
er. It  must  be  kept  in  mind,  however,  that  water,  while  per- 
forming certain  mechanical  functions,  has  a  nutrient  relation- 
ship also. 

The  most  important  mechanical  principle  governing  the  ab- 
sorption of  water  by  the  plant  is  osmosis.2  The  abstract  phe- 
nomenon should  be  clearly  in  mind  before  its  plant  relation- 
ships are  considered.  A  bag  of  collodion  (pig's  bladder  or 
parchment  paper  will  do  as  well)  is  filled  with  a  strong  solu- 

1  Gases,  such  as  carbon  dioxide,  nitrogen  and  possibly  ammonia,  may 
be  lost  from  the  soil  also. 

3  Water  may  also  be  taken  up  by  colloidal  absorption  which  is  called 
imbibition.    This  is  common  in  seeds. 


290        NATURE  AND  PROPERTIES  OF  SOILS 

tion  of  cane-sugar.  The  walls  of  such  a  bag  are  semi-permea- 
ble, that  is,  certain  materials  will  pass  through  readily  while 
others  will  pass  but  slowly.  For  example,  the  sugar  mole- 
cules penetrate  with  difficulty,  while  the  water  finds  the  walls 
of  the  bag  but  a  slight  obstacle. 

If  this  collodion  bag  with  its  sugar  solution  is  attached  to  a 
capillary  tube  and  immersed  in  pure  water,  it  at  once  becomes 
distended  and  the  liquid  will  rise  in  the  capillary  tube,  indi- 
cating an  unequal  pressure  within  the  system.  The  pressure 
develops  because  of  the  separation  of  the  pure  water  and  the 
sugar  solution  by  a  membrane  that  is  penetrated  at  different 
rates  by  the  molecules  and  ions  in  contact  with  it.  A  tendency 
towards  equalization  of  course  occurs  and,  as  the  water  moves 
in  faster  than  the  sugar  moves  out,  a  pressure  is  developed 
within  the  bag  which  becomes  apparent  by  the  rise  of  the 
liquid  in  the  capillary  tube.  Such  a  phenomenon  is  called 
osmosis  and  the  pressure  osmotic  pressure.  Such  force  prob- 
ably has  much  to  do  with  the  movement  of  plant  saps  and 
fluids.  Under  such  conditions  as  those  maintained  in  the  ex- 
periment, the  water  tends  to  move  from  the  dilute  solution  to 
the  more  concentrated  one. 

Suppose  the  collodion  bag  be  considered  as  typical  of  the 
cells,  which  form  the  feeding  surface  of  an  active  rootlet,  and 
the  sugar  solution  the  relatively  concentrated  and  partially 
colloidal  cell  contents.  The  water  outside  the  bag  will,  of 
course,  represent  the.  dilute  soil  solution  which  bathes  the  roots. 
With  such  substitutions  it  can  readily  be  seen  why  the  plant 
exerts  an  osmotic  "pull"  and  how  the  water  moves  through 
the  cell-wall.  Such  a  transfer  will  continue  until  the  move- 
ment of  the  water  in  the  soil  becomes  too  slow  for  normal 
plant  activities.    Wilting  then  occurs.    (See  Fig.  51.) 

In  alkali  soils,  where  the  soil  solution  becomes  very  concen- 
trated, the  process  above  described  may  be  reversed.  Out- 
ward   osmosis    then    occurs    and    plasmolysis1    may    result. 

1  Plasmolysis  is  a  separation  of  the  plasma  from  the  cell-wall  due  to  a 


EEMOVAL  OF  NUTRIENTS  FROM  THE  SOIL    291 

Bouyoucos  1  has  suggested  that  the  phenomena  of  wilting  may 
be  due,  at  least  partially,  to  plasmolysis  since  he  has  shown 
by  observing  the  depression  of  the  freezing  point  that  the  soil 
solution  becomes  very  concentrated  at  low  moisture  contents. 

Such  a  conception  of  water  absorption  is  simple,  yet  it  often 
leads  to  erroneous  ideas  regarding  the  intake  of  nutrients  by 
plants.  The  amount  of  any  particular  nutrient  absorbed  by 
the  plant  is  not  determined  by  the  quantity  of  water  taken  up, 
since  water  and  nutrients  enter  more  or  less  independently. 
The  large  amount  of  water  imbibed  by  the  plant,  later  to  be 
lost  by  transpiration,  cannot  be  accounted  for  on  the  basis  of 
a  very  dilute  soil  solution  and  the  necessity  of  rapid  trans- 
piration in  order  to  facilitate  the  entrance  of  sufficient  nutrient 
substance. 

153.  Absorption  of  nutrients  by  plants — diffusion. — The 
solution  in  a  normal  fertile  soil  is  not  only  rather  dilute 
in  toto  but  a  great  proportion  of  the  nutrients  therein  are  in 
the  ionic  condition.  While  both  molecules  and  ions  are  pre- 
sented to  the  absorbing  surfaces  of  the  plant,  it  is  only  the 
latter  that  penetrate  to  any  great  extent,  although  some  mate- 
rials, especially  those  of  an  organic  nature,  do  enter  in  a 
molecular  condition.  The  presence  of  water  is,  of  course,  nec- 
essary for  both  ionic  and  molecular  penetration,  but  only  as  a 
medium  for  diffusion.  Its  movement  into  the  plant  is,  there- 
fore, of  no  very  great  moment  in  the  actual  diffusion  process, 
as  the  phenomenon  is  called,  although  the  approach  of  the 
nutrients  to  the  feeding  surfaces  is  considerably  influenced  by 
capillary  activity. 

The  tendency  of  diffusion  is  to  equalize  the  concentration 
of  a  solution  as  to  the  ions  and  molecules  of  its  solute,  the 
molecules  and  ions  of  different  salts  moving  more  or  less  inde- 

loss  of  water.  It  is  a  shrinkage  of  the  protoplasm  and  when  carried 
beyond  a  certain  point  permanently  injures  the  cell. 

1  Bouyoucos,  G.  J.,  The  Freezing  Point  Method  as  a  New  Means  of 
Measuring  the  Concentration  of  the  Soil  Solution  Directly  in  the  Soil; 
Mich.  Agr.  Exp.  Sta.,  Tech.  Bui.  24,  1915. 


292        NATURE  AND  PROPERTIES  OF  SOILS 

pendently.  The  absorption  of  nutrients  by  plants,  in  its 
simplest  analysis,  is  but  a  working  out  of  this  phenomenon. 
Thus,  if  the  concentrations  of  K+  ions  is  high  in  the  soil 
solution  and  low  within  the  cell,  the  potassium  will  move 
inward  in  response  to  diffusion  forces,  providing,  of  course, 
the  ions  can  pass  through  the  cell  wall.  This  penetration  is 
entirely  independent  of  the  entrance  of  water,  as  far  as  the 


Fig.  51. — Left,  wheat  seedling  with 
soil  particles  clinging  to  root-hairs. 
Above,  root-hairs  much  enlarged. 
Root-hairs  are  simple  tube-like  pro- 
longations of  the  border  cells. 


movement  of  the  latter  is  concerned.  Moreover,  the  equaliza- 
tion of  one  ion  is  more  or  less  unrelated  to  the  concentration 
equilibrium  of  any  other.  The  osmosis  of  the  water,  on  the 
other  hand,  is  a  phenomenon  dependent  on  sum-total  concen- 
tration plus  the  semi-permeable  membrane. 

154.  Differential  diffusion. — The  intake  of  nutrients  is 
by  no  means  as  simple  as  the  above  explanation  might  lead  one 
to  assume,  due  to  the  complications  interposed  by  the  presence 
of  a  semi-permeable  membrane.  The  passage  of  ions  and  mole- 
cules through  the  cell-wall  and  the  protoplasmic  membrane 


REMOVAL  OF  NUTRIENTS  FROM  THE  SOIL    293 

may  be  a  simple  mechanical  infiltration,  although  it  is  prob- 
ably accompanied  by  a  chemical  reaction,  or  by  a  change  in 
the  colloidal  state  of  the  membrane  or  both.  Moreover,  differ- 
ent ions  and  molecules  do  not  pass  through  the  same  cell-wall 1 
with  equal  facility.  Thus,  one  kind  of  ions  may  pass  through 
very  readily  while  another  kind  may  encounter  extreme  diffi- 
culty in  responding  to  diffusion  tendencies. 

Differential  diffusion  may  be  ascribed  to  two  conditions: 
(1)  different  relationships  between  the  cell-wall  and  the  ions 
and  molecules  of  the  entering  material;  and  (2)  differences 
in  the  rate  at  which  the  entering  molecules  and  ions  are 
utilized  in  the  metabolic  activities  of  the  cell  in  particular  and 
the  plant  as  a  whole.  The  first  case  has  been  partially  ex- 
plained. If  a  compound  ionizes  into  A  and  B  ions  and  if  A 
ions,  due  to  their  relationship  to  the  colloidal  cell-wall,  enter 
more  easily,  a  residue  of  B  ions  will  be  left  in  the  soil  solution. 

The  second  case  may  be  illustrated  by  assuming  the  pres- 
ence of  potassium  chloride  in  the  soil  solution.  It  ionizes 
K+  and  CI"  ions.  Now  conceive  that  these  ions  diffuse  through 
the  cell-wall  with  equal  facility  in  response  to  equilibrium 
tendencies.  If  the  potassium  ions  are  used  by  the  cell  as 
rapidly  as  they  enter  and  are  removed  from  solution,  more 
potassium  will  be  absorbed.  This  might  continue  until  the 
potassium  ions  in  the  soil  solution  become  much  reduced  in 
number.  If  the  chlorine,  on  the  other  hand,  is  but  slightly 
utilized  by  the  plant,  little  will  be  drawn  from  the  soil  after 
the  initial  equalization.  Thus,  a  residue  of  chlorine  might  be 
left  from  this  type  of  differential  absorption.  This  applica- 
tion of  diffusion  principles  shows  the  possibility,  or  even  more, 
the  probability  of  plants  leaving  residues  in  the  soil  solution. 
What  the  residues  from  different  fertilizers  may  be  and  what 
is  the  practical  importance  of  such  differential  actions  are 
pertinent  questions. 

1  The  term  cell-wall  as  used  here  refers  to  the  cell-wall  proper  plus 
the  protoplasmic  membrane. 


294        NATURE  AND  PROPERTIES  OF  SOILS 

155.  Fertilizer  residues  may  be  developed  in  two  gen- 
eral ways:  (1)  by  selective  absorption  by  the  soil;  and  (2)  by 
differential  diffusion  into  the  plant.  Regarding  the  first  case 
(see  par.  141),  it  has  already  been  established  that  soils 
ordinarily  absorb  the  basic  ions  more  strongly  than  the  acid 
radicals,  thus  tending  to  leave  an  acid  residue  in  the  soil  solu- 
tion. Sodium  nitrate,  ammonium  sulfate,  calcium  nitrate, 
potassium  chloride  and  potassium  sulfate,  therefore,  tend  to 
produce  an  acid  residue,  when  they  are  first  added  to  a  soil. 

The  final  result,  however,  cannot  be  determined  until  the 
action  of  the  crop  is  known.  If  the  crop  especially  utilizes 
the  cation  or  basic  radical,  it  will  intensify  the  selective  ab- 
sorption of  the  soil  and  a  still  more  pronounced  acid  residue 
will  result.  This  would  be  the  case  with  ammonium  sulfate, 
potassium  sulfate,  and  potassium  chloride.  If,  however,  the 
anion  or  acid  radical  is  utilized  to  the  greater  extent,  the  ac- 
tion of  the  soil  absorption  would  be  nullified  and  an  alkaline 
residue  would  tend  to  develop.  This  is  especially  true  with 
sodium  nitrate  when  applied  in  large  amounts  over  a  term  of 
years,  the  physical  condition  of  the  soil  becoming  impaired 
due  to  the  presence  of  sodium  carbonate.1 

One  other  condition  is  possible.  If  the  plants  should  use 
the  cation  and  anion  of  a  fertilizer  salt  in  equal  proportions, 
no  residue  would  result.  This  seems  to  happen  to  an  approxi- 
mate degree  with  ammonium  nitrate,  potassium  phosphate, 
potassium  nitrate,  and  ammonium  phosphate.  Such  salts  are 
extremely  valuable  in  long-continued  experiments,  where  the 
disturbing  effects  of  fertilizer  residues  are  to  be  avoided. 
Monocalcium  phosphate,  the  important  constituent  of  acid 
phosphate,  needs  especial  consideration.  When  added  to  the 
soil,  it  immediately  reverts  to  the  tricalcium  form  if  active 
calcium  is  present.2    Even  with  the  large  amount  of  gypsum 

1  Hall,  A.  D.,  The  Effect  of  the  Long  Continued  Use  of  Sodium  Nitrate 
on  the  Constitution  of  the  Soil;  Trans.  Chem.  Soc.  (London),  Vol.  85, 
pp.  950-971,  1904. 

2CaH4(P04)2  +  2CaH2(C03)2  =  Ca3(P04)2  +  4H20  +  4C02 


REMOVAL  OF  NUTRIENTS  FROM  THE  SOIL    295 

carried  by  acid  phosphate,  the  effect  does  not  seem  to  be 
towards  acidity  even  after  long  periods  of  application.1 

This  discussion,  brief  as  it  is,  brings  out  a  little  studied 
phase  of  crop  and  fertilizer  interaction.  How  the  plant  util- 
izes a  particular  fertilizer  after  it  is  once  in  the  soil,  what 
residues  are  left,  and  the  importance  of  such  residues,  are 
questions  of  fundamental  concern.  The  possibility  of  plants 
influencing  the  soil  and  the  fertilizers  added,  as  well  as  the 
soil  and  fertilizer  influencing  the  crop,  is  well  worth  attention. 

156.  Do  plants  directly  aid  in  the  preparation  of  their 
nutrients? — The  conception  commonly  held  regarding  the 
plant  is  that  its  direct  relation  to  the  soil  is  more  or  less 
passive.  Indirectly,  of  course,  it  may  exert  a  considerable 
influence  on  the  availability  of  the  nutrients.  In  view  of  the 
knowledge  regarding  fertilizer  residues  and  the  new  concepts 
as  to  possible  root  exudates,  the  idea  that  the  plant  may 
directly  aid  in  the  preparation  of  its  own  nutrients  is  becom- 
ing more  and  more  plausible. 

Such  influences,  if  recognized,  might  occur  in  three  ways: 
(1)  through  the  action  of  carbon  dioxide,  known  to  be  given 
off  in  large  amounts  by  roots;  (2)  through  the  influence  of 
organic  and  inorganic  acids  other  than  carbonic  acid;  and 
(3)  by  catalytic  agents,  enzymic  or  non-enzymic. 

In  a  rich,  moist  soil  the  number  of  root-hairs  is  very  large 
and  the  relationship  between  the  rootlets  and  the  soil  particles 
very  intimate.  When  in  contact  with  a  particle  of  soil  or 
colloidal  complex,  the  root-hair  in  many  cases  almost  incloses 
it,  and  by  means  of  its  mucilaginous  wall  forms  a  contact  so 
close  as  to  make  the  solution  held  between  the  particle  and  the 
cell-wall  distinct  from  that  in  the  soil  proper.  Carbon  dioxide, 
excreted  under  such  conditions,  may  assume  a  solvent  power 
entirely  unique  and  independent  of  the  amount  produced. 

1  Conner,  S.  D.,  Acid  Soils  and  the  Effect  of  Acid  Phosphate  and 
Other  Fertilisers  Upon  Them;  Jour.  Ind.  and  Eng.  Chem.,  Vol.  8,  No.  1, 
pp.  35-40,  Jan.  1916. 


296        NATURE  AND  PROPERTIES  OF  SOILS 

The  plant  might  thus  facilitate  special  conditions  and  aid  ma- 
terially in  the  preparation  of  its  own  nutrients. 

Sachs,1  and  later  other  investigators,  grew  plants  of  various 
kinds  in  soil  and  other  media  in  which  was  placed  a  slab  of 
polished  marble  or  dolomite  or  calcium  phosphate,  covered 
with  a  layer  of  washed  sand.  After  the  plants  had  made 
sufficient  growth  the  slabs  were  removed,  and  on  the  surfaces 
were  found  corroded  tracings,  corresponding  to  the  lines  of 
contact  between  the  rootlets  and  the  minerals. 

Czapek  2  repeated  the  experiments  of  Sachs,  using  plates 
of  gypsum  mixed  with  the  ground  mineral  that  he  wished 
to  test,  and  this  mixture  he  spread  over  a  glass  plate.  Cza- 
pek found  that,  while  plates  of  calcium  carbonate  and  of 
calcium  phosphate  were  corroded  by  the  roots,  plates  of  alu- 
minum phosphate  were  not.  He  concludes  that  if  the  tracings 
are  due  to  acids  excreted  by  the  roots,  these  acids  must 
be  those  that  have  no  solvent  action  on  aluminum  phos- 
phate. This  would  limit  the  excreted  acids  to  carbonic, 
acetic,  proprionic,  and  butyric.  By  means  of  micro-chem- 
ical analyses  of  the  exudations  of  root-hairs  grown  in  a 
water-saturated  atmosphere,  Czapek  found  potassium,  mag- 
nesium, calcium,  phosphorus,  and  chlorine  in  the  exudate.  He 
concludes  that  the  solvent  action  of  roots  is  due  to  acid  salts 
of  mineral  acids,  particularly  acid  potassium  phosphate.  He 
has  not  proved,  however,  that  the  exudations  were  not  from 
dead  root-hairs  or  from  the  dead  cells  of  the  root  cap.  In 
either  case  they  would  have  some  solvent  action,  but  whether 
sufficient  to  make  them  of  importance  is  doubtful.  This  ob- 
jection makes  the  possible  exudation  of  organic  and  inorganic 
acids  somewhat  questionable. 

Molisch3  found  that  root-hairs  secrete  a  substance  having 

1  Sachs,  J.,  Auflosung  des  Marmors  durch  Mais-Wurzlen;  Bot.  Zeitung, 
18  Jahrgang,  Seite  117-119,  1860. 

"Czapek,  J.,  Zur  Lehre  von  den  Wurzelausscheidung ;  Jahrb.  f.  Wiss. 
Bot.,  Band  29,  Seite  321-390,  1896. 

8  Molisch,  H.,  uber  Wurzelausscheidungen  und  deren  EinwirTcung  auf 


REMOVAL  OF  NUTRIENTS  FROM  THE  SOIL    297 

properties  corresponding  to  those  of  an  oxidizing  enzyme. 
His  work  has  been  repeated  by  others,  who  have  failed  to  ob- 
tain similar  results,  but  lately  Schreiner  and  Reed 1  have 
demonstrated  an  oxidizing  action  of  roots  that  is  apparently 
due  to  a  peroxidase.  Oxidation  alone,  however,  would  hardly 
suffice  to  account  for  the  solvent  action  accompanying  the  de- 
velopment of  roots,  although  it  is  doubtless  an  important 
function  and  useful  in  other  ways. 

Schreiner  and  Sullivan 2  have  demonstrated  the  presence 
of  reducing  substances  in  media  in  which  plants  were  grow- 
ing. This  work  has  recently  been  corroborated  by  Lyon  and 
Wilson,3  working  with  maize,  oats,  peas,  and  vetch.  They 
found  that  the  solutions  in  which  the  plants  had  been  growing 
exhibited  both  reducing  and  oxidizing  phenomena.  Reducing 
substances  were  always  present,  but  whether  oxidizing  mate- 
rials were  so  consistently  produced  could  not  be  definitely 
decided.  The  peroxidases  were  rendered  inactive  by  boiling 
the  solutions.  The  reducing  substances  did  not  always  disap- 
pear with  such  treatment.  This  would  throw  some  doubt  upon 
the  enzymic  character  of  the  reducing  materials  and  suggest 
that  non-enzymic  catalytic  exudates  are  a  possibility. 

The  interstices  between  the  larger  particles  of  a  normal 
soil  are  at  least  partially  filled  with  colloidal  material  of  a 
more  or  less  gel-like  nature.  Moreover,  the  surfaces  of  some 
soil  grains  may  be  somewhat  coated  with  the  same  material. 
Roots  of  growing  plants  have  been  found  to  cause  coagula- 
tion of  at  least  some  colloids,  possibly  by  leaving  an  acid 
residue  in  the  nutrient  solution  by  reason  of  the  selective 

Organische  Substanzen;  Sitzungsber.  Akad.  Wiss.  Wien-Math.  Nat.,  Band 
96,  Seite  84-109,  1888.  Abstract  in  Chem.  Centrlb.  f.  Agr.  Chem.,  Band 
17,  Seite  428,  1888. 

1  Schreiner,  Oswald,  and  Reed,  H.  S.,  Studies  on  the  Oxidising  Powers 
of  Boots;  Bot.  Gazette,  Vol.  47,  p.  355,  1909. 

2  Schreiner,  O.,  and  Sullivan,  M.  K.,  Studies  in  Soil  Oxidation;  U.  S. 
Dept.  Agr.,  Bur.  Soils,  Bui.  73,  1910. 

•Lyon,  T.  L.,  and  Wilson,  J.  K.,  Liberation  of  Organic  Matter  by 
Boots  of  Growing  Plants;  Cornell  Agr.  Exp.  Sta.,  Memoir  40,  July,  1921. 


298        NATURE  AND  PROPERTIES  OF  SOILS 

absorption  of  bases  and  rejection  of  the  acid  radicals  of  the 
dissolved  salts.  It  is  conceivable  that  the  root-hairs,  by  re- 
moving bases  from  the  solution  existing  between  the  cell-wall 
and  the  colloidal  covering  of  the  soil  particle,  may  cause 
coagulation  of  the  colloidal  matter  and  thus  liberate  the  nu- 
trient materials  held  by  absorption.  The  liberated  material, 
being  of  a  readily  soluble  nature,  would  be  taken  up  by  the 
solution  between  the  rootlet  and  the  soil  particle,  from  which 
the  root-hair  could  readily  absorb  it.  Such  an  hypothesis 
would  account  for  the  ability  of  plants  to  obtain  a  quantity 
of  nutrients  far  in  excess  of  that  accounted  for  by  the  solvent 
action  of  pure  water,  and  even  beyond  what  many  investi- 
gators are  willing  to  attribute  to  the  solvent  action  of  water 
charged  with  carbon  dioxide. 

157.  The  present  status  of  the  question. — The  available 
evidence  on  excretion  of  acids  other  than  carbonic  by  the 
roots  of  plants  does  not  admit  of  any  very  satisfactory  conclu- 
sion as  to  their  relative  importance  in  the  acquisition  of  plant 
nutrients.  There  can  be  no  doubt,  however,  that  carbon 
dioxide  resulting  from  root  exudation  and  from  decomposi- 
tion of  organic  matter  in  the  soil  plays  a  very  prominent  part 
in  this  operation.  The  very  large  quantity  of  carbon  dioxide 
in  the  soil,  amounting  in  some  cases  to  nearly  10  per  cent,  of 
the  soil  air,  or  several  hundred  times  that  of  the  atmospheric 
air,  must  aid  greatly  in  dissolving  the  soil  particles. 

Whatever  may  be  the  concentration  of  the  soil-water,  it 
seems  probable  that  the  liquid  that  is  found  where  the  root- 
hair  comes  in  contact  with  the  soil  particle,  and  that  is  sepa- 
rated, in  part  at  least,  from  the  remainder  of  the  soil-water, 
must  have  a  composition  different  from  that  found  elsewhere 
in  the  soil.  Many  plants  grown  in  solutions  of  nutritive  salts 
have  few  or  no  root-hairs,  but  absorb  through  the  epidermal 
tissue  of  the  roots.  The  special  modification  by  which  the 
root-hairs  come  in  intimate  contact  with  the  soil  particle  and 
almost  surround  it,  indcates  a  direct  relation  between  the 


KEMOVAL  OF  NUTRIENTS  FROM  THE  SOIL    299 

soil  particles  and  the  plant,  as  well  as  between  the  soil-water 
and  the  plant.  Such  a  condition  complicates  in  no  small 
degree  the  practical  questions  of  soil  management  and  plant 
nutrition. 

158.  Why  crops  vary  in  their  ability  to  thrive  on  dif- 
ferent soils. — It  is  very  commonly  recognized  that  crops  of 
different  kinds  vary  in  their  ability  to  obtain  nourishment 
from  the  soil.  The  difference  between  the  nitrogen,  phosphoric 
acid,  potash,  and  lime  taken  up  by  an  average  corn  crop  and 
a  wheat  crop  of  average  size  is  striking.  The  terms  "weak 
feeders' '  and  "strong  feeders,"  so  often  heard,  indicate  the 
practical  field  relationships.  Aside  from  the  fact  that  crops 
do  not  all  need  the  same  quantities  of  nutrients  these  differ- 
ences in  ability  to  grow  normally  on  different  soils  may  be 
due  either  to  (1)  a  larger  absorbing  system  or  (2)  a  more 
active  absorptive  capacity. 

Plants  with  large  root  systems  may  be  expected  to  absorb 
greater  amounts,  not  only  of  water  but  of  nutrients  also.1 
Such  a  development  is  especially  important  in  time  of  drought 
and  in  addition  gives  the  plant  a  greater  area  from  which  to 
draw  nutrients.  Water,  as  well  as  nutrients,  does  not  move 
through  any  great  distance  towards  the  imbibing  and  ab- 
sorbing surfaces.  Root  development,  while  of  some  impor- 
tance in  explaining  the  differences  in  the  feeding  capacities 
of  plants,  is  probably  by  no  means  as  important  as  differences 
in  the  absorption  activity. 

The  absorptive  activity  of  a  plant  under  any  given  condi- 
tion of  soil,  climate,  and  stage  of  growth  depends  on:  (1)  the 
concentration  and  composition  of  the  cell-sap;  (2)  the  char- 
acter of  the  cell- wall;  (3)  the  activity  of  the  cell  in  elabo- 
rating and  removing  from  solution  the  materials  absorbed; 
(4)  the  extent  to  which  exudates — whether  these  be  carbon 

1  Gile,  P.  L.,  and  Carrero,  P.  L.,  Adsorption  of  Nutrients  as  Affected 
by  the  Number  of  Boots  Supplied  with  the  Nutrient;  Jour.  Agr.  Res., 
Vol.  IX,  No.  3,  pp.  73-95,  1917. 


300        NATURE  AND  PROPERTIES  OF  SOILS 

dioxide,  organic  or  mineral  acids  and  their  salts  or  enzymes 
— act  on  the  colloidal  and  non-colloidal  soil  constituents;  and 
(5)  synergistic  relationships  in  the  soil  solution  or  the  cell- 
wall. 

The  concentration  and  composition  of  the  cell-sap  deter- 
mines not  only  the  osmotic  relationship  but  has  much  to  do 
with  diffusion  tendencies.  The  ability  of  the  plant  to  obtain 
water  and  nutrients  is  thus  directly  affected  by  such  condi- 
tions. The  character  of  the  cell-wall  has  of  course  an  im- 
portant influence  on  such  phenomena.  If  the  cell-wall  is 
easily  penetrated,  it  may  greatly  facilitate  the  absorbing  ca- 
pacity of  the  plant.  If  it  is  slowly  penetrated  or  exerts  spe- 
cial differential  influences,  it  might  have  a  great  deal  to  do 
with  the  differences  observed  between  certain  plants.  The 
character  of  the  cell-wall  has  already  been  shown  to  be  in- 
volved in  the  development  of  certain  residues  in  the  soil. 

The  rate  at  which  materials  are  utilized  within  the  plant 
is  also  a  factor.  If  ions  or  molecules  are  used  rapidly  and 
thus  removed  from  solution,  the  diffusion  of  similar  ions  and 
molecules  is  hastened.  Such  activity  would  also  influence 
osmotic  relationships  to  a  marked  extent.  This  has  already 
been  discussed  under  differential  diffusion. 

It  is  readily  conceivable  that  exudates,  insofar  as  they  are 
capable  of  directly  affecting  the  solubility  of  nutrients,  might 
produce  marked  differences  between  plants  as  far  as  their 
absorbing  activities  are  concerned.  A  crop  producing  active 
exudates  of  any  kind  should  be  able,  other  conditions  being 
equal,  to  grow  to  better  advantage,  especially  on  a  soil  in 
which  the  necessary  nutrients  are  somewhat  unavailable. 

The  absorption  of  electrolytes  by  plants  seems  to  be  influ- 
enced by  the  presence  of  other  nutrient  ions.  True  1  has 
shown  that  K+  ions  when  accompanied  by  Ca++  ions  are  readily 
absorbed  by  the  seedlings  of  certain  plants.    When  the  same 

1  True,  E.  H.,  The  Function  of  Calcium  in  the  Nutrition  of  Seedlings; 
Jour.  Amer.  Soc.  Agron.,  Vol.  13,  No.  3,  pp.  91-107,  1921. 


KEMOVAL  OF  NUTRIENTS  FROM  THE  SOIL    301 

concentration  of  potassium  is  offered  in  single  solution,  this 
nutrient  is  more  or  less  neglected  by  the  seedlings.  This  rela- 
tionship, by  which  the  calcium  ions  make  the  potassium  physi- 
ologically available,  is  spoken  of  by  True  as  synergism  x  and 
probably  has  a  great  deal  to  do  with  the  penetration  of  nu- 
trient ions  into  the  plant.  It  is  no  doubt  of  considerable  im- 
portance in  acid  soils  where  the  active  calcium  is  low. 

159.  The  absorptive  capacity  of  different  crops. — 
Cereals  have  the  power  of  utilizing  the  potassium  and  phos- 
phorus of  the  soil  to  a  considerable  degree,  but  they  generally 
require  fertilization  with  nitrogen  salts.  Most  of  the  cereals, 
such  as  wheat,  rye,  oats,  and  barley,  take  up  the  principal 
part  of  their  nitrogen  early  in  the  season,  before  the  nitrifica- 
tion processes  are  sufficiently  operative  to  furnish  a  large 
supply  of  nitrogen ;  hence  nitrogen  is  the  fertilizer  constituent 
that  usually  gives  good  results,  and  should  be  added  in  a 
soluble  form.  Wheat,  in  particular,  needs  a  large  amount  of 
available  nitrogen  early  in  its  spring  growth.  Since  it  is  a 
" delicate  feeder,"  it  does  best  after  a  cultivated  crop  or  a 
fallow,  by  which  the  nitrogen  has  been  converted  into  a  soluble 
form.  Oats  can  make  better  use  of  the  soil  nutrients  and  do 
not  require  so  much  manuring.  Maize  is  a  very  "coarse 
feeder/'  and,  while  it  removes  a  large  quantity  of  plant  nu- 
trients from  the  soil,  it  does  not  require  that  this  shall  be 
added  in  a  soluble  form.  Farm  and  other  slowly  acting  ma- 
nures may  well  be  applied  for  the  maize  crop.  The  long 
growing  period  required  by  maize  gives  it  opportunity  to 
utilize  the  nitrogen  as  it  becomes  available  during  the  summer, 
when  ammonification  and  nitrification  are  active.  Phosphorus 
is  the  substance  usually  most  needed  by  maize. 

Grasses,  when  in  meadow  or  in  pasture,  are  greatly  bene- 
fited by  manures.  They  are  less  vigorous  '  *  feeders ' '  than  the 
cereals,  have  shorter  roots,  and,  when  allowed  to  grow  for 
more  than  one  year,  the  lack  of  aeration  in  the  soil  causes  the 

1  The  term  is  used  here  in  the  sense  of  cooperation. 


302        NATURE  AND  PROPERTIES  OF  SOILS 

decomposition  of  soil  organic  matter  to  decrease.  There  is 
usually  a  more  active  fixation  of  nitrogen  in  grass  lands  than 
in  cultivated  lands,  but  this  nitrogen  becomes  available  very 
slowly. 

Different  soils  and  climatic  conditions  necessitate  varied 
methods  of  manuring  for  grass.  Farm  manures  may  well  be 
applied  to  meadows  in  all  situations,  while  the  use  of  available 
nitrogen  in  commercial  fertilizers  is  generally  profitable. 

Most  of  the  leguminous  crops  are  deep-rooted  and  are  vigo- 
rous "feeders."  Their  ability  to  take  nitrogen  from  the  air 
makes  the  use  of  that  fertilizer  constituent  unnecessary  ex- 
cept in  a  few  instances,  such  as  young  alfalfa  on  poor  soil, 
where  a  small  application  of  nitrate  of  soda  is  usually  bene- 
ficial. Phosphoric  acid  and  often  lime  are  the  substances  most 
beneficial  to  legumes  on  most  soils. 

Many  crops  will  utilize  very  large  quantities  of  nutrients 
if  they  are  in  a  form  in  which  they  can  be  used.  Phosphates 
and  nitrogen  are  the  substances  generally  required,  the  latter 
especially  by  beets  and  carrots.  In  growing  vegetables  the 
object  is  to  produce  a  rapid  growth  of  leaves  and  stalks  rather 
than  seeds,  and  often  this  growth  is  made  very  early  in  the 
season.  As  a  consequence  a  soluble  form  of  nitrogen  is  very 
desirable.  Farm  manure  should  also  have  a  prominent  part 
in  the  treatment,  as  it  keeps  the  soil  in  a  mechanical  condition 
favorable  to  the  retention  of  moisture,  which  vegetables  re- 
quire in  large  amounts,  and  it  also  supplies  needed  fertility. 
The  very  intensive  method  of  culture  employed  in  the  produc- 
tion of  vegetables  necessitates  the  use  of  much  greater  quan- 
tities of  manures  than  are  used  for  field  crops,  and  the  great 
value  of  the  product  justifies  the  practice. 

160.  Quantities  of  nutrients  removed  by  crops. — The 
utilization  of  nutrient  substances  by  crops  is  a  constant  source 
of  loss  of  fertility  to  agricultural  soils.  In  a  state  of  nature 
the  loss  in  this  way  is  comparatively  small,  as  the  native  vege- 
tation falls  on  the  ground,  and  in  the  process  of  decomposi- 


REMOVAL  OF  NUTRIENTS  FROM  THE  SOIL    303 

tion  the  ash  is  almost  entirely  returned,  while  there  is  a  large 
gain  of  organic  matter  and  often  an  increase  in  nitrogen  as 
well.  Under  natural  conditions  the  soil  usually  increases  in 
fertility;  for,  while  there  is  some  loss  through  drainage  and 
other  sources,  this  is  more  than  counterbalanced  by  the  action 
of  the  natural  agencies  of  disintegration  and  decomposition, 
while  the  fixation  of  atmospheric  nitrogen  affords  a  constant, 
though  small,  supply  of  that  important  soil  ingredient. 

When  land  is  placed  under  cultivation  a  very  different 
condition  is  presented.  Crops  are  removed  and  only  par- 
tially returned  at  best  to  the  soil  as  manure  and  crop  resi- 
due. A  certain  proportion  of  the  soil  nutrients  are,  therefore, 
permanently  withdrawn.  The  point  of  vital  importance, 
however,  is  that  only  a  part  of  the  total  supply  of  soil  con- 
stituents will  ever  become  available,  the  portion  withdrawn 
each  year  by  cropping  being  a  more  serious  consideration 
than  is  generally  supposed. 

The  following  table,  computed  by  Warington,1  shows  the 
quantities  of  nitrogen,  potash,  phosphoric  acid,  lime  and  sulfur 
trioxide  2  removed  from  an  acre  of  soil  by  some  of  the  common 
crops.    The  entire  harvested  crop  is  included. 

Table  LXVI 


Crop 

Wheat 

Barley 

Oats 

Maize 

Meadow  Hay 
Red  Clover .  < 
Potatoes. . . . 


Yield 


30  bushels 
40  bushels 
45  bushels 
30  bushels 
V/2  tons 
2  tons 
6       tons 


Ash 

N 

K20 

CaO 

PA 

(lbs.) 

(LBS.) 

(lbs.) 

(lbs.) 

(lbs.) 

172 

48 

28.8 

9.2 

21.1 

157 

48 

35.7 

9.2 

20.7 

191 

55 

46.1 

11.6 

19.4 

121 

43 

36.3 

... 

18.0 

203 

49 

50.9 

32.1 

12.3 

258 

102 

83.4 

90.1 

24.9 

127 

47 

76.5 

3.4 

21.5 

so3 

(LBS.) 


15.7 
14.3 
19.7 
12.0 
11.3 
15.4 
11.5 


1  Warington,  E.,  Chemistry  of  the  Farm;  pp.  64-65,  London,  1894. 

"From  Hart,  E.  B.,  and  Peterson,  W.  H.,  Sulphur  Requirements  of 
Farm  Crops  in  Relation  to  the  Soil  and  Air  Supply;  Wis.  Agr.  Exp.  Sta., 
Res.  Bui.  14,  1911. 


304        NATURE  AND  PROPERTIES  OF  SOILS 

Before  the  question  of  possible  soil  exhaustion  can  be  dis- 
cussed adequately,  the  losses  of  nutrients  in  the  drainage 
water  must  be  considered  as  another  source  of  loss  in  addition 
to  the  cropping  influences  already  noticed. 

161.  Qualitative  composition  of  drainage  water. — In 
theory,  at  least,  the  qualitative  composition  of  drainage  water 
should  be  the  same  as  that  of  the  soil  solution ;  that  is,  in  it 
should  be  found  all  of  the  common  bases  and  acid  radicals. 
Actually,  however,  due  to  the  absorptive  power  of  the  soil, 
certain  constituents  appear  in  very  slight  amounts.  Phos- 
phorus, for  example,  often  occurs  in  drainage  only  in  traces, 
as  do  the  nitrites,  ammonia,  and  carbonates.  The  principal 
bases  lost  by  leaching  are  calcium,  magnesium,  potassium, 
and  sodium.  The  important  acid  radicals  of  drainage  water 
are  the  nitrates,  chlorides,  sulfates,  and  bicarbonates. 

As  might  be  expected,  the  constituents  appearing  in  drain- 
age are  extremely  variable  not  only  when  different  soils  are 
compared  but  also  within  the  same  soil  at  different  periods. 
Phosphorus  may  be  leached  from  some  soils  in  measureable 
quantities,  while  from  others  the  amount  may  be  negligible. 
Nitrate  nitrogen  is  usually  an  important  constituent  in  all 
drainage  water  during  the  summer,  especially  that  from  a  bare 
soil.  In  the  winter  and  early  spring  nitrates  decrease  in 
amount.  The  method  of  soil  treatment  as  to  cultivation,  ma- 
nuring, liming,  or  fertilizing  may  also  markedly  influence  the 
qualitative  composition  of  the  water  draining  from  field  soil. 

162.  Quantitative  composition  of  drainage  water. — While 
but  little  reliable  data  regarding  the  composition  and 
especially  the  concentration  of  the  soil  solution  are  available 
at  the  present  time,  much  exact  information  has  been  obtained 
regarding  drainage  water.  The  concentration  of  drainage 
water  is  much  lower  than  that  of  the  soil  solution  and  much 
less  variable.  The  total  concentration  seems  to  be  governed 
more  by  the  amount  of  water  leaching  through  than  by  any 
other  factor.     Other  seasonal  conditions  of  course  come  into 


REMOVAL  OF  NUTRIENTS  FROM  THE  SOIL    305 

play.  In  total  concentration,  drainage  water  seldom  exceeds 
500  parts  per  million.  It  is  thus  much  more  dilute  than  the 
average  soil  solution.  This  difference  holds  for  the  separate 
constituents  as  well  as  for  the  concentration  in  toto. 

The  following  data,  as  compiled  by  Hall,1  give  some  idea 
of  the  quantitative  composition  of  the  drainage  water  from 
the  clay  loam  soil  of  the  Rothamsted  Experimental  Farm. 
The  drainage  water  was  obtained  from  tile  drains,  a  line  of 
which  extended  under  each  of  the  variously  treated  plats. 
The  data  is  a  mean  of  five  collections,  1866  to  1868. 

Table  LXVII 


Treatment 

Parts    per 

Million    Based   on    Solution 

N205 

NH3 

p2o5 

K20 

CaO 

MgO 

Na20 

Fe203 

CI 

S03 

Si02 

No  manure 

Farm  manure,  14 

3.9 

16.1 
5.1 

16.9 

18.4 

.12 

.16 
.13 

.27 

.24 

.63 

.91 

.17 

1.7 

5.4 
5.4 

2.7 

4.1 

98.1 

147.4 
124.3 

197.3 

118.1 

5.1 

4.9 
6.4 

8.9 

5.9 

6.0 

13.7 
11.7 

10.6 

56.1 

5.7 

2.6 
4.4 

2.7 

5.1 

10.7 

20.7 
11.1 

39.4 

12.0 

24.7 

106.1 
66.3 

89.7 

41.0 

10.9 
35  7 

Minerals  2    only. . 
Minerals  plus  600 

lbs.    (NH4)2S04 
Minerals  plus  550 

lbs.     NaN03..., 

15.4 
20.9 
10.6 

It  is  immediately  noticeable  that  ammoniacal  nitrogen  and 
phosphoric  acid  are  lost  in  drainage  to  but  a  slight  degree. 
Calcium  appears  in  the  highest  concentration  with  sulfur 
next.  Nitrates  and  potash  are  present  in  appreciable  quan- 
tities but  are  quite  variable. 

The  influence  of  treatment  is  particularly  obvious  on  the 
parts  per  million  of  nitrate  nitrogen,  lime,  and  sulfur  appear- 
ing in  the  drainage,  the  addition  of  farm  manure  increasing 
all  of  these  constituents  as  well  as  the  concentration  of  the 
potash,  soda,  and  chlorine.  The  application  of  sodium  nitrate 
increased  the  nitrate  nitrogen  as  well  as  the  soda,  potash,  and 

1  Hall,  A.  D.,  The  Booh  of  the  Bothamsted  Experiments;  pp.  237-239, 
New  York,  1917. 

2  By  minerals  are  meant  the  phosphoric  acid,  potash,  lime,  and  other 
constituents  left  as  ash  when  plants  are  burned. 


306         NATURE  AND  PROPERTIES  OF  SOILS 

lime.  The  two  latter  constituents  are  probably  liberated  by 
basic  exchange.  The  addition  of  any  fertilizer  seems  espe- 
cially to  increase  the  lime  in  the  drainage  water.  This  is  prob- 
ably due  to  the  development  of  acid  fertilizer  residues.  In 
general,  it  seems  that  the  more  productive  the  soil  and  the 
heavier  the  fertilization,  the  higher  the  concentration  of  the 
constituents  in  the  drainage  water. 

It  is  not  always  the  case,  however,  that  a  manured  soil 
loses  more  nutrient  material  than  an  unfertilized  one.  Ger- 
lach x  reports  experiments  with  soil  tanks  at  the  Bromberg 
Institute  of  Agriculture,  in  which  five  soils  rationally  fertil- 
ized yielded  larger  crops  and  lost  in  the  main  less  nitrogen 
and  lime  in  the  drainage  water  than  the  same  soils  unmanured. 
The  loss  of  potash  was  slightly  greater  from  the  manured  than 
from  the  unmanured  soils.  Apparently  the  stimulation  that 
the  plants  received  from  the  fertilizer  enabled  them  to  make 
such  a  good  growth  that  they  absorbed  more  soluble  nitrogen 
and  lime  in  excess  of  the  unfertilized  plants  than  was  added 
in  the  fertilizer,  and  nearly  as  much  potash. 

The  most  serious  losses  of  plant  nutrients  in  drainage  are 
those  of  the  nitrogen  and  calcium,  both  of  which  losses  are  to 
a  certain  extent  unavoidable.  These  losses  are  also  very 
closely  related,  rising  and  falling  together.  Nitrogen  is  lost 
as  the  nitrate  while  the  calcium  is  leached  out  due  to  the 
presence  of  the  bicarbonate  and  nitrate  radicals.  While  loss 
of  lime  goes  on  continually,  it  is  of  necessity  particularly 
large  during  periods  of  rapid  nitrate  accumulation.  Nitrogen 
is  a  high-priced  fertilizer  constituent,  while  a  continued  loss 
of  lime  tends  to  produce  soil  acidity.  About  the  only  means 
of  conserving  either  of  these  constituents  is  to  maintain  a  crop 
on  the  soil,  especially  during  the  warmer  seasons. 

1  Gerlach,  M.,  uber  die  durch  Sickerwasser  dem  Boden  Entzogenen 
Menge  Wasser  und  Nahrstoffe;  Illus.  Landw.  Zeitung,  30  Jahrgang, 
Heft  95,  Seite  871-881,  1910.  Also,  Untersuchungen  uber  die  Menge  und 
Zusammensetzung  der  Sickerwasser;  Mitt.  K.  W.  Inst.  f.  Landw.  in 
Bromberg,  Band  3,  Seite  351-381,  1910. 


REMOVAL  OF  NUTRIENTS  FROM  THE  SOIL    307 

163.  Quantities  of  nutrients  removed  by  drainage  and 
cropping. — Now  that  an  adequate  conception  has-been  pre- 
sented regarding  the  composition  of  soil  drainage  water  and 
also  of  the  nutrients  removed  by  cropping,  it  is  interesting  to 
note  what  the  combined  result  may  be  on  the  same  soil.  Such 
information  can  be  obtained  only  in  a  few  instances.  The 
following  data  from  the  lysimeters  at  the  Cornell  Experiment 
Station  1  are  valuable  in  this  respect.2  The  soil  used  was  a 
Dunkirk  silty  clay  loam. 


Table  LXVIII 

AVERAGE    ANNUAL    LOSS    OF    NUTRIENTS    BY    PERCOLATION 
CROPPING.        CORNELL  LYSIMETER  TANKS. 
AVERAGE  OF  10  YEARS. 


AND 


Condition 

Pounds  to  the  Acre  per  Year 

N 

P306 

K20 

CaO 

S08 

Drainage  losses 

Bare 

Rotation .... 

Grass 

Crop  removal 

Bare 

Rotation .... 

Grass 

Total  loss 

Bare 

Rotation .... 

Grass 

69.0 
7.3 
2.5 

70.5 
54.4 

69.0 
77.8 
56.9 

43.5 
28.6 

43.5 

28.6 

86.4 
68.7 
74.0 

105.4 
74.0 

86.4 
174.1 
158.0 

557.0 
345.9 
363.8 

24.3 

12.8 

557.0 
370.2 
376.6 

132.0 
108.5 
111.0 

41.0 

29.2 

132.0 
149.5 
140.2 

1  Unpublished  data,  Cornell  Agr.  Exp.  Sta.,  Ithaca,  N.  Y. 

2  A  study  was  made  at  the  New  Jersey  Experiment  Station  of  the 
nitrogen  losses  from  a  loam  soil  in  cylinders  under  a  five-year  rotation 
of  corn,  oats,  wheat  and  timothy  for  20  years,  treated  in  various  ways 
as  to  lime,  manure  and  fertilizers.  The  average  loss  of  nitrogen  from 
the  surface  ten  inches  of  soil  for  15  years  was  103  pounds  annually 
due  to  cropping  and  leaching.  Data  were  obtained  by  analyzing  the 
soil  and  the  crops.  Lipman,  J.  G.,  and  Blair,  A.  W.,  Nitrogen  Under 
Intensive  Cropping;  Soil  Sci.,  Vol.  XII,  No.  1,  pp.  1-16,  July,  1921. 


308        NATURE  AND  PROPERTIES  OF  SOILS 

The  first  outstanding  feature  of  the  above  table  is  the  con- 
trol on  drainage  losses  exerted  by  cropping.  The  loss  of 
nitrate  nitrogen  is  reduced  to  an  exceptionally  low  figure, 
while  the  saving  of  potash,  sulfur,  and  lime  is  quite  apprecia- 
ble. No  phosphoric  acid  is  lost  even  from  the  bare  soil.  The 
losses  due  to  cropping  and  leaching  combined  from  a  planted 
soil  are  generally  but  little  greater  than  the  drainage  losses 
alone  from  a  soil  kept  continuously  bare  except  in  the  cases 
of  the  phosphoric  acid  and  the  potash. 

The  next  point  of  interest  is  the  difference  in  the  nutrients 
removed  by  a  rotation  of  crops,  such  as  maize,  oats,  wheat, 
and  hay  as  compared  with  permanent  meadow.  The  latter, 
although  absorbing  less  nutrients  than  the  rotation  crops, 
exert  as  marked  a  conserving  effect  on  the  nutrients  appear- 
ing in  the  drainage  as  do  the  crops  in  rotation.  The  compara- 
tive removal  of  nutrients  from  the  soil  by  cropping  and  leach- 
ing are  well  shown  by  the  following  diagram,  in  which  the 
weight  of  the  symbols  indicates  where  the  loss  of  any  par- 
ticular nutrient  is  the  greater. 

RELATIVE  LOSSES  OF  NUTRIENTS  FROM  A  PLANTED  SOIL  THROUGH 
CROPPING  AND  DRAINAGE 


Nitrogen 

Phos- 
phoric 
acid 

Potash 

Lime 

Sulfur 
Trioxide 

Cropping  loss 

Drainage  loss .... 

N 

N 

PA 

PA 

K,0 
K'O 

CaO 
CaO 

so8 
so3 

164.  Possible  exhaustion  of  the  soil. — It  is  interesting  at 
this  point  to  compare  the  amounts  of  nutrients  removed  an- 
nually from  a  soil  cropped  in  rotation  with  the  amounts  which 
are  present  in  an  average  soil  to  the  depth  of  four  feet.  As- 
suming reasonable  figures  for  the  pounds  of  sulfur  trioxide, 
lime,  phosphoric  acid,  nitrogen,  and  potash  and  considering 
that  these  nutrients  are  wholly  available,  the  following  sig- 


REMOVAL  OF  NUTRIENTS  FROM  THE  SOIL    309 

nificant  data  are  obtained.  The  losses  of  nutrients  by  drain- 
age and  rotation  cropping  are  from  the  figures  already  quoted 
regarding  the  Cornell  lysimeter  soils. 

Table  LXIX 

SHOWING  THE  NUMBER  OF  YEARS  A  SOIL  TO  THE  DEPTH  OP  POUR 

FEET    WOULD    SUPPLY    NUTRIENTS    FOR    CROP    GROWTH, 

PROVIDING  THAT  ALL  OF  THESE  CONSTITUENTS 

WERE  UNIFORMLY  AVAILABLE 


Constituents 

Pounds  to 

the  Depth  of 

Four  Feet 

Pounds 

Removed 

Annually  by 

Cropping  and 

Drainage 

Years 

S03 

12,000 
85,000 
16,000 
15,000 
250,000 

84.5  l 
370.2 
43.5 
46.8 » 
174.1 

142 

CaO 

P20, 

229 
367 

N 

303 

K20 

1,435 

While  the  subsoil  supplies  large  amounts  of  plant  nutrients, 
it  must  be  remembered  that  only  a  small  proportion  of  the 
soil  constituents,  especially  the  phosphoric  acid  and  potash, 
ever  become  available  either  in  surface  or  subsoil.  Moreover, 
crop  yields  decrease  as  the  nutrients,  even  those  most  readily 
available,  are  reduced.  The  above  figures  for  duration  of 
crop  growth  are,  as  a  consequence,  merely  conventional  but 
they  indicate  the  probability  of  even  a  very  fertile  soil  be- 
coming quickly  exhausted. 

Moreover,  when  it  is  considered  that  the  soil  must  be  de- 
pended on  to  furnish  food  for  humanity  and  domestic  animals 
as  long  as  they  shall  continue  to  inhabit  the  earth,  the  supply 
of  plant  nutrients  becomes  a  matter  of  grave  concern. 

The  visible  sources  of  supply,  to  replace  or  supplement 

1  Sixty-five  pounds  of  S03  are  added  an  acre  each  year  in  rain- 
water while  31  pounds  of  N  are  added  yearly  to  the  acre  in  rain  and 
through  the  free-fixing  activity  of  organisms  (pars.  222,  236  and  238). 


310        NATURE  AND  PROPERTIES  OF  SOILS 

those  in  the  soils  now  cultivated,  are,  for  the  mineral  sub- 
stances, the  subsoil  and  the  natural  deposits  of  phosphates, 
potash  salts,  and  limestone;  and  for  nitrogen,  deposits  of 
nitrate,  the  by-products  of  coal  distillation,  and  the  nitrogen 
of  the  atmosphere.  The  last  of  these  is  inexhaustible,  and  the 
exhaustion  of  the  nitrogen  supply,  which  a  few  years  ago  was 
thought  to  be  a  matter  of  less  than  half  a  century,  has  now 
ceased  to  cause  any  apprehension. 

The  conservation  or  extension  of  the  supply  of  mineral 
nutrients  is  now  of  extreme  importance.  The  utilization  of 
city  refuse  and  the  discovery  of  new  mineral  deposits  are 
developments  well  within  the  range  of  possibility,  but  neither 
of  these  promises  to  afford  more  than  partial  relief.  The 
utilization  of  the  subsoil  through  the  gradual  removal  by  nat- 
ural agencies  of  the  topsoil  will,  without  doubt,  tend  constantly 
to  renew  the  supply.  The  removal  of  topsoil  by  wind  and 
water  erosion  is,  even  on  level  land,  a  very  considerable  factor. 
The  large  amount  of  sediment  carried  in  streams  immediately 
after  a  rain,  especially  in  summer,  gives  some  idea  of  the  ex- 
tent of  this  shifting.  This  affects  chiefly  the  surface  soil,  and 
thereby  brings  the  subsoil  into  the  range  of  root  action. 

There  is  little  doubt  that  a  moderate  supply  of  plant  nu- 
trients will  always  be  available  in  most  soils,  but  for  progres- 
sive agriculture  the  use  of  green-manures,  legumes  and  farm 
manures  must  be  supplemented  by  judicious  and  economical 
application  of  lime  and  certain  fertilizer  constituents. 


CHAPTER  XVI 
CHEMICAL  ANALYSIS  OF  SOILS 

No  phase  of  soil  science  has  received  as  much  popular  rec- 
ognition as  chemical  analysis,  nor  is  any  other  technical  soil 
procedure  so  little  understood  in  general  and  at  the  same 
time  so  greatly  overrated.  Many  persons  feel  that  a  soil 
analysis  should  completely  solve  the  many  problems,  both 
theoretical  and  practical,  regarding  the  economic  management 
of  the  soil,  especially  as  to  its  fertilizer  needs.  In  the  light 
of  such  general  misunderstanding  in  regard  to  the  research 
and  applied  value  of  chemistry  to  soils,  a  consideration  of  the 
question  seems  opportune  at  this  point,  especially  as  the  dis- 
cussion of  the  phenomena  of  absorption  and  the  characteristics 
of  the  soil  solution  have  just  been  presented. 

For  convenience  in  treatment,  chemical  analyses,  as  applied 
to  soils,  may  be  grouped  under  two  heads — total  or  bulk 
analyses  and  partial  or  extraction  methods.  In  the  former  the 
total  amount  of  certain  constituents  are  determined  regard- 
less of  their  chemical  combinations  and  character.  In  the 
latter  group  of  methods  only  a  portion  of  certain  important 
materials  are  removed  and  analyzed,  the  chemical  combina- 
tion being  to  a  certain  extent  a  factor  in  the  amount  of  any 
constituent  extracted. 

165.    Bulk    analysis — organic    carbon    and    nitrogen.1 — 

*The  sampling  of  the  soil  is  an  important  consideration  in  any 
analytical  work.  The  sample  should  be  representative  and  is  best 
taken  with  a  soil  auger.  In  sampling  small  areas,  such  as  plats,  a  num- 
ber of  borings  are  usually  made  to  the  depths  required  and  thoroughly 
mixed.     This  composite  is  quartered  until  a  sample  of  the  required  size 

311 


312 


NATUEE  AND  PROPERTIES  OF  SOILS 


The  methods  of  determining  the  amount  of  organic  matter  in 
any  soil  have  already  been  discussed  (par.  60),  the  conclusion 
being  that  the  figure  for  organic  carbon,  or  this  figure  multi- 
plied by  1.724,  was  the  most  reliable  indication  of  the  organic 
content  of  a  soil.  The  bomb  method  is  cited  as  one  of  the 
more  suitable  procedures  for  obtaining 
the  organic  soil  carbon. 

The  method  for  estimating  the  soil 
humus,  although  it  is  not  a  bulk  method, 
should  be  considered  at  this  point  because 
of  its  close  relationship  to  the  determina- 
tion of  organic  carbon.  The  modified 
Grandeau  procedure  (par.  61)  is  used  for 
humus  estimation  and  is  supposed  to  dis- 
tinguish between  the  more  active  and  less 
active  organic  matter.  Of  the  two 
methods  the  determination  of  organic  car- 
bon is  by  far  the  more  accurate.  As 
there  is  also  some  doubt  about  the  com- 
parative activity  of  the  material  ex- 
tracted by  the  Grandeau  procedure  the 
figure  for  organic  carbon  seems  in  general 
the  more  significant  and  it  is  the  deter- 
mination usually  made.  The  estimation 
of  soil  humus  may,  therefore,  be  con- 
sidered as  a  chemical  method  of  sec- 
ondary   importance    except    in     special 


Pig.  52.— Auger  used 
in  the  field  exam- 
ination of  soil  and 
in  taking  samples. 
Note  modified  cut- 
ting edge. 


cases. 


The  total  nitrogen  of  the  soil  is  determined  by  either  the 
Kjeldahl,  the  modified  Kjeldahl,  or  by  the  Gunning  method. 
The  determination  of  nitrogen  is  such  a  common  laboratory 


is  obtained.  Where  large  areas  are  involved,  as  in  the  case  of  a  soil 
survey,  only  one  sample,  representative  of  the  soil  type  being  studied,  is 
usually  taken. 


CHEMICAL  ANALYSIS  OF  SOILS  313 

procedure  that  it  is  worth  while  to  consider  the  principles  in- 
volved.1 About  10  grams  of  dry  soil  are  placed  in  a  Kjeldahl 
flask  with  about  30  c.c.  of  strong  sulfuric  acid  and  0.7  gram 
of  mercuric  oxide  or  its  equivalent  in  metallic  mercury.  The 
mixture  is  boiled  vigorously  until  the  solution  is  clear.  The 
flask  is  then  removed  from  the  flame  and,  while  hot,  potassium 
permanganate  is  added  in  small  quantities  to  complete  the 
oxidation  until,  after  shaking,  the  liquid  remains  a  green  or 
purple  color.  The  nitrogen  of  the  soil,  no  matter  what  has 
been  its  combination,  is  now  in  the  form  of  ammonium  sulfate 
[(NH4)2SOJ,  the  mercury  acting  as  a  catalytic  agent  and 
the  permanganate  as  an  oxidizer. 

After  cooling,  the  contents  of  the  flask  are  diluted  with 
about  200  c.c.  of  water,  zinc  dust  or  a  few  pieces  of  granu- 
lated zinc  are  added  to  prevent  bumping  and  25  c.c.  of 
potassium  sulfid  are  poured  in  with  shaking.  Next  a  sodium 
hydroxide  solution,  sufficient  in  amount  to  neutralize  the  acid, 
is  carefully  poured  down  the  side  of  the  flask.  The  flask  is 
then  connected  with  a  condenser  and  the  contents  cautiously 
mixed  by  shaking.  The  ammonia  set  free  by  the  alkali  is  dis- 
tilled over  into  a  standard  acid,  the  excess  acid  being  titrated 
with  a  standard  alkali,  using  a  suitable  indicator.  When  the 
amount  of  standard  acid  neutralized  is  known,  the  amount  of 
nitrogen,  which  has  passed  over  in  the  form  of  ammonia,  may 
be  calculated  and  expressed  as  a  percentage,  based  on  the 
original  dry  sample  of  soil.     (See  Fig.  53.) 

1  The  method  described  above  is  the  Kjeldahl  method.  See  Official 
and  Tentative  Methods  of  Analysis  of  the  Assoc.  Official  Agr.  Chemists, 
p.  314,  1920. 

This  method  does  not  determine  the  nitrogen  in  the  nitrate  form. 
If  this  is  desired  a  modified  procedure  must  be  followed.  As  the 
nitrate  nitrogen  in  most  soil  is  low  compared  to  the  nitrogen  in  other 
combinations,  the  objection  just  made  to  the  regular  Kjeldahl  method 
is  not  serious. 

Snyder,  E.  S.,  Determination  of  Total  Nitrogen  in  Soils  Containing 
Bather  Large  Amounts  of  Nitrates;  Soil  Sci.,  Vol.  VI,  No.  6,  pp.  487- 
490,  1918. 


314         NATURE  AND  PROPERTIES  OF  SOILS 

166.  Bulk  analysis — complete  solution  of  the  soil. — By 
the  use  of  hydrofluoric  acid  or  by  fusion  with  potassium  and 
sodium  carbonate,  the  entire  soil  mass  may  be  decomposed  and 


Fig.  53. — Front  and  side  view  of  a  Kjeldahl  distilling  battery.  (A), 
Kjeldahl  flask;  (B),  trap;  (C),  condenser  tank;  (D),  receiving  flask 
containing  standard  acid  and  (E),  Bunsen  burner. 


its  constituents  determined.1     The  amount  of  lime  (CaO)  or 
any  other  constituent,2  may  thus  be  expressed  in  percentage 

1  Wiley,  Harvey  W.,  Principles  and  Practices  of  Agricultural  Chemical 
Analysis,  Vol.  1,  pp.  398-399,  1906. 

a  Schollenberger    presents    some    interesting    data   regarding    the   pro- 


CHEMICAL  ANALYSIS  OF  SOILS 


315 


based  on  dry  soil  or  in  pounds  to  the  acre  to  any  suitable 
depth.  This  method  gives  only  the  total  of  any  constituent 
and  tells  nothing  regarding  its  availability  to  crops,  although 
a  marked  deficiency  in  any  element  may  thus  be  detected.  A 
rock  will  often  show  greater  amounts  of  the  mineral  elements 
than  a  fertile  soil.1 

167.  Partial  analysis  of  the  soil  for  mineral  constitu- 
ents.— When  it  was  realized  that  a  bulk  analysis  of  the  soil, 
especially  for  the  mineral  constituents,  gave  no  information 
as  to  the  availability  of  certain  elements  or  as  to  the  fertilizer 
needs  of  the  soil,  extraction  methods  were  devised.  Such 
methods,  of  whatever  character  they  may  be,  are  designed  to 

portion  of  organic  and  inorganic  phosphorus  in  Ohio  soils.  The  figures 
are  an  average  of  twelve  types. 


Soils 

Total 
PA 

Organic  P206 

as  Per  Cent 

of  Total 

Total 

N 

Cultivated 

0-  7  inches 

% 

.0433 
.0345 

.0587 
.0381 

% 

34 

20 

24 
21 

% 
.14 

7-15  inches 

.07 

Virgin 

0-  7  inches 

.19 

7-15  inches 

.08 

Schollenberger,  C.  J.,  Organic  Phosphorus  Content  of  Ohio  Soils; 
Soil  Sci.,  Vol.  X.  No.  2,  pp.  127-141,  1920. 

For  methods  of  determining  organic  phosphorus,  see  Potter,  R.  S., 
The  Organic  Phosphorus  of  Soil;  Soil  Sci.,  Vol.  II,  No.  4,  pp.  291- 
298,  1916. 

,Eost,  C.  O.,  The  Determination  of  Soil  Phosphorus;  Soil  Sci.,  Vol. 
IV,  No.  4,  pp.  295-311,  1917. 

Potter,  R.  S.,  and  Snyder,  R.  S.,  The  Organic  Phosphorus  of  Soil; 
Soil  Sci.,  Vol.  VI,  No.  5,  pp.  321-332,  1918. 

Schollenberger,  C.  J.,  Organic  Phosphorus  of  Soil:  Experimental  Work 
on  Methods  for  Extraction  and  Determination;  Soil  Sci.,  Vol.  VI,  No.  5, 
pp.  365-395,  1918. 

1  Sulfur  is  determined  by  a  separate  method.  Official  and  Tentative 
Methods  of  Analysis  of  the  Assoc,  of  Official  Agr.  Chemists,  p.  317,  1920. 

See  also,  Hart,  E.  B.,  and  Peterson,  W.  H.,  Sulphur  Requirements  of 
Farm  Crops  in  Relation  to  the  Soil  and  Air  Supply;  Wis.  Agr.  Exp. 
Sta.,  Res.  Bui.  14,  1911. 


316 


NATURE  AND  PROPERTIES  OF  SOILS 


give  information  regarding  the  availability  of  the  plant  nutri- 
ents within  the  soil.  They  may  be  listed  under  three  heads: 
(1)  digestion  with  strong  acids,  (2)  digestion  with  dilute 
acids,  and  (3)  extraction  with  water.  These  methods  will  be 
discussed  in  the  order  mentioned. 

168.  Digestion  with  strong  acids. — While  surfuric,  ni- 
tric, and  hydrochloric  acids  have  all  been  used  as  solvents,1 
the  one  most  commonly  employed  is  hydrochloric  acid  of 
1.115  specific  gravity.2  It  has  been  used  to  such  an  ex- 
tent that  it  may  be  considered  the  standard  solvent,  and  a 
statement  of  a  chemical  analysis  of  a  soil  in  this  country  may 
be  considered  as  based  on  this  solvent  unless  otherwise  stated. 

An  analysis  by  this  method  is  supposed  to  show  the  propor- 
tion of  nutrient  materials  in  a  soil  that  is  in  a  condition  to 
be  used  ultimately  by  plants  at  the  time  when  the  analysis  is 
made.  The  nutrient  materials  that  are  not  dissolved  by 
treatment  with  hydrochloric  acid  are  assumed  to  be  in  a 
condition  in  whch  plants  cannot  use  them.  The  difficulty 
with  this  assumption  is  that,  while  treatment  with  hydro- 
chloric acid  of  a  given  strength  marks  a  definite  point  in  the 
solubility  of  the  compounds  in  the  soil,  it  does  not  bear  a 
uniform  relation  to  the  natural  processes  by  which  these 
compounds  become  available  to  plants. 

This  method  is  not  only  arbitrary  but  it  is  artificial  as  well. 

1  The  following  analyses  of  the  same  soil  quoted  from  Snyder  are 
interesting  in  this  regard.  Snyder,  H.,  Soils;  Minn.  Agr.  Exp.  Sta., 
Bui.  41,  p.  66,  1895. 


Extract 

Total 

K20 

CaO 

MgO 

P206 

S03 

HC1 

% 
18.80 
16.55 
19.55 

% 
.42 
.30 
.52 

% 

.55 
.30 
.53 

% 
.40 
.32 
.52 

% 
.23 
.23 

.26 

% 
08 

HN03 

H2S04.... 

.08 
.10 

2  Official  and  Provisional  Methods  of  Analysis;  U.  S.  Dept.  Agr.,  Bur. 
Chem.,  Bui.  107  (revised),  pp.  14-18,  1908. 


CHEMICAL  ANALYSIS  OF  SOILS 


317 


While  it  is  supposed  to  measure  the  permanent  fertility1  of 
a  soil,  there  is  no  reason  to  suppose  that  there  is  any  rela- 
tionship between  the  nutrients  extracted  by  a  strong  acid 
in  the  laboratory  and  the  amounts  of  the  same  constituents 
absorbed  by  crops  over  a  period  of  fifty  or  one  hundred  years. 
Moreover,  productivity  is  not  necessarily  controlled  by  the 
amounts  of  available  nutrients  in  a  soil.  This  further  vitiates 
the  data  obtained  by  such  an  analysis. 

Snyder2  has  analyzed  a  number  of  Minnesota  soils  by 
means  of  digestion  with  strong  hydrochloric  acid,  decompos- 
ing the  acid-insoluble  residues  by  fusion  and  determining 
their  composition.  Veitch  3  has  analyzed  certain  Maryland 
soils  by  the  hydrochloric  acid  method  and  by  means  of  com- 
plete solution.  A  few  examples  are  given  below  to  show  how 
soils  may  vary  in  the  solubility  of  their  constituents  in  strong 
hydrochloric  acid: 

Table  LXX 

PERCENTAGE  OF  SOIL   CONSTITUENTS  INSOLUBLE  IN 
HQ,   SP.  GR.    1.115 


Soils 


Minnesota  (Snyder) 

Fair  Haven 

Holden 

Experiment  Station.  . 
Maryland  (Veitch) 

Columbia 

Chesapeake 

Hudson  River  Shale. 


K20 

CaO 

MgO 

P206 

94 

25 

58 

40 

81 

61 

76 

45 

83 

41 

36 

18 

95 

90 

34 

66 

67 

82 

29 

15 

73 

37 

28 

0 

so3 


74 
90 
20 


169.  Digestion  with  dilute  acids. — A  great  number  of 
different  acids  have  been  used  in  a  dilute  condition  for  ex- 

1  Fertility  is  used  here  in  the  sense  of  potential  productivity,  the 
nutrients  in  the  soil  being  considered  as  the  controlling  factor. 

2 Snyder,  Harry,  Soils;  Minn.  Agr.  Exp.  Sta.,  Bui.  41,  p.  35,  1895. 

*  Veitch,  F.  P.,  The  Chemical  Composition  of  Maryland  Soils;  Md. 
Agr.  Exp.  Sta.,  Bui.  70,  p.  103,  1901. 


318        NATURE  AND  PROPERTIES  OF  SOILS 

tracting  soils,  the  idea  being  in  every  case  to  determine  the 
amount  of  the  mineral  nutrients  immediately  available  to 
crops.  The  scope  is  thus  narrower  than  in  the  digestion  with 
strong  acids,  by  which  the  permanent  fertility  is  sought. 

Two  acids  have  been  commonly  utilized  in  the  extraction  of 
soils  with  dilute  solvents :  one  per  cent,  citric  acid  proposed  by 
Dyer,1  and  one-fifth  normal  nitric  acid.2  Dyer  adopted  the 
one-per-cent.  strength  as  the  result  of  an  investigation  in  which 
he  determined  the  acidity  of  the  juices  in  the  roots  of  over 
one  hundred  species  or  varieties  of  plants  representing  twenty 
different  natural  orders.  The  implication  is  that  plants  pro- 
duce a  solvent  action  on  a  soil  in  proportion  to  the  acidity  of 
their  juices,  but  an  examination  of  Dyer's  figures  does  not 
show  that  the  size  of  the  crop  ordinarily  produced  by  the  plants 
would  in  many  cases  correspond  to  the  acidity  of  their  juices. 
Thus,  of  the  Cruciferae,  the  horse-radish  has  several  times  the 
acidity  of  the  Swedish  turnip  or  of  the  field  cabbage,  although 
the  crop  produced  by  the  former  is  much  less  than  that  of  the 
latter  two. 

Dyer's  method  gave  results  on  Rothamsted  soils  that  en- 
abled him  to  estimate  their  relative  productivity.  On  other 
soils  and  in  the  hands  of  other  investigators,  however,  the 
method  is  unsatisfactory.  In  soils  rich  in  calcium  and  low  in 
iron  and  aluminum,  it  may  often  show  the  amounts  of  easily 
soluble  phosphoric  acid  and  potash. 

In  ease  of  manipulation,  the  fifth  normal  nitric  acid  is 
preferable  to  the  one-per-cent.  citric  acid,  which  is  rather 
tedious  to  work  with.  It  has  been  utilized  nearly  as  exten- 
sively in  this  country  as  has  the  latter  in  Great  Britain.  Its 
use  has  been  confined  largely  to  the  determination  of  the 
readily  available  phosphoric  acid  and  potash  in  the  soil,  as 

1  Dyer,  Bernard,  On  the  Analytical  Determination  of  Probably  Avail- 
able "Mineral"  Plant  Food  in  Soils;  Jour.  Chem.  Soc,  Vol  LXV, 
pp.  115-167,  1894. 

*  Official  and  Provisional  Methods  of  Analysis;  U.  S.  Dept.  Agr.,  Bur. 
Chem.,  Bui.  107  (revised),  p.  18,  1908. 


1 


CHEMICAL  ANALYSIS  OF  SOILS  319 

has  the  citric  acid  method.  It  is  obvious  that  some  materials 
are  more  readily  soluble  than  others,  and  for  that  reason  the 
method  will  distinguish  between  phosphorus  and  potassium 
in  different  forms.  The  calcium  phosphates  are  supposed  to 
be  entirely  soluble  in  this  strength  of  acid.  According  to 
Fraps,1  it  dissolves  iron  and  aluminum  phosphates  to  only  a 
slight  extent,  thus  distinguishing  between  these  forms  of  phos- 
phorus and  calcium  phosphate.  Fraps  finds  also  that  no 
potassium  is  removed  from  orthoclase  and  microcline,  that  less 
than  10  per  cent,  is  dissolved  from  glauconite  and  biotite,  and 
that  from  15  to  60  per  cent,  is  dissolved  from  muscovite, 
nephelite,  leucite,  apophyllite,  and  phillipsite,  minerals  known 
to  be  rather  easily  available. 

There  are  several  factors,  however,  that  make  the  use  of 
one-fifth  normal  nitric  acid  an  uncertain  guide  to  the  avail- 
able phosphoric  acid  and  potash  in  the  soil.  When  a  soil  is 
treated  with  the  acid,  some  of  it  is  neutralized  by  the  reac- 
tions that  result  and  thus  its  strength  is  lessened.  This  may 
have  no  relation  to  the  quantities  of  phosphoric  acid  or  potash 
dissolved.  Some  analysts  correct  for  the  neutralization  and 
some  do  not.  Again,  as  with  concentrated  hydrochloric  acid, 
the  degree  of  solubility  of  the  soil  constituents  in  the  nitric 
acid  may  not  correspond  with  the  ability  of  the  plant  to  ob- 
tain these  substances.  With  this,  as  with  the  other  methods 
discussed,  the  objection  holds  that  the  results  cannot  be  taken 
as  an  infallible  guide  to  the  productiveness  of  a  soil,  or  to  its 
fertilizer  needs.  The  artificial  extraction  of  a  soil  in  the 
laboratory  cannot  be  expected  to  simulate  the  action  of  a 
crop  even  for  one  year. 

170.  Extraction  with  water. — As  carbon  dioxide  is  a 
universal  constituent  of  the  water  of  the  soil,  and  without 

1  Fraps,  G.  S.,  Active  Phosphoric  Acid  and  Its  Relation  to  the  Needs 
of  the  Soil  for  Phosphoric  Acid  in  Pot  Experiments;  Tex.  Agr.  Exp. 
Sta.,  Bui.  126,  pp.  7-72,  1909. 

Also,  The  Active  Potash  of  the  Soil  and  Its  Relation  to  Pot  Experi- 
ments; Tex.  Agr.  Exp.  Sta.,  Bui.  145,  pp.  5-39,  1912. 


320        NATURE  AND  PROPERTIES  OF  SOILS 

doubt  a  potent  factor  in  the  decomposition  of  the  mineral 
matter,  it  has  been  proposed  to  use  a  solution  of  carbon  diox- 
ide as  a  solvent  in  soil  analysis.  The  amounts  of  soil  con- 
stituents taken  up  by  this  solvent  are  much  less  than  are  taken 
up  by  any  of  the  others  heretofore  mentioned,  but  all  mineral 
substances  used  by  plants  are  soluble  in  it  to  some  extent. 
The  amount  of  phosphoric  acid  is  so  small  as  to  make  its 
detection  by  the  gravimetric  method  difficult.  Like  other 
methods  employing  very  weak  solvents,  this  is  open  to  the 
objection  that  much  of  the  material  dissolved  cannot  be  re- 
moved because  of  the  absorptive  power  of  the  soil,  and  as  this 
varies  with  the  character  of  the  soil,  adequate  comparisons 
cannot  be  made.  Water  charged  with  carbon  dioxide  has  been 
very  largely  replaced  by  pure  water  in  making  such  extrac- 
tions. 

When  soil  is  digested  with  distilled  water,  all  the  mineral 
substances  used  by  plants  are  dissolved  from  it,  but  in  very 
small  quantities.  It  has  been  proposed  to  employ  this  extract 
for  soil  analysis  on  the  ground  that  it  is  a  natural  solvent 
and  dissolves  only  those  nutrients  in  a  condition  to  be  used 
by  plants.  By  determining  the  moisture  content  of  the  soil 
and  using  a  known  quantity  of  water  for  the  extraction,  the 
parts  per  million  of  the  extracted  nutrients  may  be  expressed 
on  the  basis  of  the  dry  soil  or  of  the  solution.  The  aqueous 
extract  does  not  by  any  means  contain  the  entire  quantity  of 
nutrients  which  were  in  the  soil  solution  and  is  not  an  exact 
measure  of  the  fertility  in  this  form.  Absorption  holds  back 
an  undetermined  and  variable  quantity  of  the  important  con- 
stituents and  thus  vitiates  the  method,  especally  for  compar- 
ing different  soils.  The  method,  however,  is  very  valuable  for 
comparing  the  same  soil  at  different  times,  especially  as  re- 
gards the  nitrates.  The  nitrate  radical  is  not  absorbed  to  any 
great  degree  by  the  soil  and  presents  a  very  fair  measure  of 
the  concentration  of  the  soil  solution  as  far  as  this  constituent 
is  concerned. 


CHEMICAL  ANALYSIS  OF  SOILS 


321 


The  water  extract  method  generally  followed  in  this  country 
is  that  established  by  the  Bureau  of  Soils.  One  hundred 
grams  of  soil  are  mixed  with  500  cubic  centimeters  of  water 
and  stirred  for  three  minutes.  After  standing  twenty  minutes 
the  supernatant  liquid  is  filtered  through  a  Pasteur-Chamber- 
land  filter  under  pressure.  It  is  then  ready  for  analysis. 
Colometric  and  turbidity  methods  are  usually  employed  in  de- 
termining the  amounts  of  the  constituents  removed.1  The 
method  is  of  greatest  use  in  estimating  the  nitrate  content  of 
soils. 

The  quantity  of  extracted  material  depends  on  the  absorp- 
tive properties  of  the  soil,  on  the  amount  of  water  used  in  the 
extraction,  and  on  the  number  of  extractions.  Analyses  of 
the  aqueous  extract  of  a  clay  and  of  a  sandy  soil  from  the 
Cornell  University  farm  serve  to  illustrate  the  greater  reten- 
tive power  of  the  former  for  nitrates.  Sodium  nitrate  was 
applied  to  a  clay  soil  and  to  a  sandy  loam  soil  at  the  rate  of 
640  pounds  to  the  acre.  Analyses  of  aqueous  extracts  some 
ninety  days  later  showed  the  following : 

Table  LXXI 


Kind  of  Soil 

Fertilizer 

Nitrates  in  Soil 

(Parts  per 

million) 

Clay 

Sodium    nitrate 
No  fertilizer 
Sodium    nitrate 
No  fertilizer 

7.8 

Clay 

1.8 

Sandy  loam 

Sandy  loam 

150.0 
29.7 

There  was  apparently  a  much  greater  retention  of  nitrate 
by  the  clay  soil,  as  shown  by  a  comparison  of  the  fertilized 
and  unfertilized  plats  on  both  soils. 


1  Schreiner,  O.,  and  Failyer,  G.  H.,  Colorimetric,  Turbidity  and  Titra- 
tion Methods  Used  in  Soil  Investigations;  U.  S.  Dept.  Agr.,  Bur.  Soils, 
Bui.  31,  1906. 


322 


NATURE  AND  PROPERTIES  OF  SOILS 


Schulze  1  extracted  a  rich  soil  by  slowly  leaching  one  kilo 
with  pure  water,  one  liter  of  water  passing  through  in  twenty- 
four  hours.  The  extract  for  each  twenty-four  hours  was 
analyzed  every  day  for  a  period  of  six  days.  The  total  amounts 
dissolved  during  each  period  were  as  follows : 


Table  LXXII 


Successive  Extraction 

Total  Matter 
Dissolved 

GRAMS 

Volatile 
grams 

Inorganic 
grams 

First 

Second 

.535 
.120 
.261 
.203 
.260 
.200 

.340 
.057 
.101 
.083 
.082 
.077 

.195 
.063 

Third 

.160 

Fourfh 

.120 

Fifth 

.178 

Sixth 

.123 

It  will  be  noticed  that  the  dissolved  matter,  both  organic 
and  inorganic,  fell  off  markedly  after  the  first  extraction. 
Later  extractions  were  doubtless  supplied  largely  from  the 
substances  held  by  absorption,  which  gradually  diffused  into 
the  water  extract  as  the  tendency  to  maintain  equilibrium  of 
the  solution  overcame  the  absorptive  action.  With  the  re- 
moval of  the  absorbed  substances  the  equilibrium  between 
the  absorption  and  solution  surfaces  and  the  surrounding  so- 
lution is  disturbed,  diffusion  and  solution  are  increased,  and 
more  material  gradually  passes  from  the  soil  into  the  solution. 
In  this  way,  a  more  or  less  uniform  and  continuous  extraction 
is  mantained. 

In  spite  of  the  obvious  defects  of  the  water  extraction 
method  the  work  of  Hoagland,  Burd  and  Stewart 2  seems  to 
indicate  that  such  data,  if  obtained  over  an  extended  period, 

1  Schulze,  F.,  fiber  den  Phosphorsaure-Gehalt  des  Wasser-Auszugs  der 
Ackererde;  Landw.  Vers.  Stat.,  Band  6,  Seite  409-412,  1864. 

2  Burd,  J.  S.,  Water  Extractions  of  Soils  as  a  Criteria  of  their  Crop- 


CHEMICAL  ANALYSIS  OF  SOILS  323 

are  a  good  comparative  measure  of  the  concentration  and 
composition  of  the  soil  solution  (see  par.  145).  They  also  con- 
sider water  extractions  as  criteria  of  the  crop-producing  power 
of  a  soil  so  studied.  The  practical  value  of  such  a  method  as 
a  means  of  estimating  fertility  is,  however,  somewhat  ques- 
tionable, since  much  time  and  labor  are  required  to  make  the 
necessary  extractions  and  analyses  before  conclusions  at  all 
reliable  may  be  drawn. 

171.  Fertility  evaluation  by  means  of  chemical  analyses. 
— The  important  part  that  chemistry  plays  in  soil  investiga- 
tion and  research  should  not  be  overlooked.  Nor  can  a  satis- 
factory presentation  of  soil  phenomena,  whether  with  a  tech- 
nical or  an  applied  bearing,  be  made  without  the  use  of  some 
chemistry.  Chemistry,  in  fact,  is  the  fundamental  science 
that  is  most  utilized  in  soil  study. 

In  spite  of  these  relationships,  the  value  of  chemistry  in  the 
direct  solution  of  practical  fertility  problems  is  neither  abso- 
lute nor  final.  The  objections  already  raised  to  the  digestion 
of  the  soil,  either  with  concentrated  or  dilute  acids,  shows  the 
inadequacy  of  these  methods  so  far  as  practical  problems  are 
concerned. 

Of  all  the  chemical  analyses  discussed  those  that  have  to  do 
with  the  determination  of  organic  carbon,  total  nitrogen,  total 
calcium  and  phosphoric  acid  are  of  outstanding  value.  Or- 
ganic matter  is  such  an  important  soil  constituent  that  a 
knowledge  of  its  amount  cannot  fail  to  throw  much  light  on 
the  physical  and  chemical  condition  of  the  soil.  Much  of  the 
soil  nitrogen  is  carried  by  the  organic  matter  and  becomes 
available  in  much  larger  proportion  than  do  the  mineral 
nutrients.     An   analysis  for  total  nitrogen   is,   therefore,    a 

Producing  Power;  Jour.  Agr.  Kes.,  Vol.  XII,  No.  6,  pp.  297-309,  1918. 

Stewart,  G.  E.,  Effect  of  Season  and  Crop  Growth  in  Modifying  the 
Soil  Extract;  Jour.  Agr.  Res.,  Vol.  XII,  No.  6,  pp.  311-368,  1918. 

Hoagland,  D.  R.,  The  Freezing  Point  Method  as  an  Index  of  Varia- 
tions in  the  Soil  Solution  Due  to  Season  and  Crop;  Jour.  Agr.  Res., 
Vol.  XII,  No.  6,  pp.  369-395,  1918. 


324        NATURE  AND  PROPERTIES  OF  SOILS 

fairly  reliable  guide  in  some  cases  to  the  fertility  of  the  soil 
under  specific  consideration. 

Although  the  relationship  of  organic  matter  and  nitrogen 
to  soil  fertility  is  so  close  that  certain  generalized  tables  x  may 
be  cited  for  the  interpretation  of  chemical  data,  no  close  cor- 
relation is  possible,  especially  where  soils  of  markedly  different 
character  are  compared.  So  many  other  factors  may  enter 
that  practically  no  opinion  can  be  formed  regarding  the  prod- 
uctivity of  a  soil  unless  other  and  more  detailed  data  are 
available. 

An  interesting  example  of  where  the  nitrogen  content  fails 
to  indicate  the  relative  fertility  of  two  soils  is  found  in  certain 
unpublished  data  from  the  Cornell  Agricultural  Experiment 
Station.  Two  soils  are  being  studied  in  the  lysimeter  tanks — 
Dunkirk  silty  clay  loam  and  Volusia  silt  loam.  In  Table 
LXXIII  is  given  the  nitrogen  and  calcium  content  of  these 
soils  and  the  pounds  of  nitrogen  removed  to  the  acre  by  maize, 
oats,  and  barley,  respectively,  for  the  years  1915,  1916,  and 
1917.  The  treatment  and  handling  of  the  soils  compared  has 
been  the  same. 

While  the  nitrogen,  phosphoric  acid  and  potash  contents  of 
these  soils  are  about  the  same,  a  marked  difference  is  noted  in 
their  productivity.  This  may  be  due,  at  least  partially,  to 
the  calcium  content,  which  is  rather  high  in  the  Dunkirk, 
especially  in  the  subsoil.     In  comparing  soils  over  wide  areas 

1  The  following  tentative  classification  of  soils  on  the  basis  of  their 
percentages  of  organic  matter  and  nitrogen  is  offered  for  generalized 
field  use: 


Description 


Percentage  of 
Organic  Matter 


Percentage  of 

Nitrogen 


Low 

Medium. . . 
High  .... 
Very  high 


.0-  3.0 

3.0-  6.0 

6.0-10.0 

above  10.0 


.00-  .10 

.10-  .25 

.25-  .40 

above  .40 


CHEMICAL  ANALYSIS  OF  SOILS 


325 


Table  LXXIII 

THE  PERCENTAGES  OF  NITROGEN  AND  CALCIUM  IN  THE  DUNKIRK 

SILTY  CLAY  LOAM  AND  THE  VOLUSIA  SILT  LOAM  AND  THE 

NITROGEN  REMOVED  BY  CERTAIN  CROPS.    CORNELL 

LYSI  METER    TANKS. 


Soils 

CaO 

% 

N 
% 

Pounds  of  N  Eemoved 
per  Acre 

maize 
1915 

OATS 

1916 

BARLEY 
1917 

Dunkirk  silty  clay  loam. . . 
Firstfoot 

.340 

.280 

.490 

1.530 

.230 
.165 
.260 
.365 

.134 
.062 
.064 
.054 

.145 
.052 
.059 
.050 

53.6 

28.3 

62.3 
21.7 

44.0 

Second  foot 

Third  foot 

Fourth  foot 

Volusia  silt  loam 

18.8 

First  foot 

Second  foot 

Third  foot 

Fourth  foot 

and  in  a  general  way  there  is  often  some  correlation  between 
the  amount  of  calcium  present  and  the  productivity.  In 
humid  regions  soils  high  in  lime  are  usually  fertile.  Within 
certain  limits,  therefore,  calcium  becomes  significant  in  fer- 
tility studies.1 

Some  idea  concerning  the  relative  value  of  the  various  chem- 
ical methods,  especially  those  dealing  with  potash,  lime,  phos- 
phoric acid,  and  magnesia,  may  perhaps  be  obtained  by  com- 
paring actual  data.    Burd  2  has  analyzed  a  number  of  soils, 


^hedd,  O.  M.,  A  Proposed  Method  for  the  Estimation  of  Total 
Calcium  in  Soils  and  the  Significance  of  this  Element  in  Soil  Fertility; 
Soil  Sci.,  Vol.  X,  No.  1,  pp.  1-14,  1920. 

aBurd,  J.  S.,  Chemical  Criteria,  Crop  Production  and  Physical  Classi- 
fication in  Two  Soil  Classes;  Soil  Sci.,  Vol.  V,  No.  6,  pp.  405-419,  1918. 


326 


NATURE  AND  PROPERTIES  OF  SOILS 


some  good,  some  poor,  by  several  different  methods.     Repre- 
sentative figures  are  given  below : 

Table  LXXIV 

CHEMICAL  COMPOSITION  OF  A  GOOD  AND  A  POOR  SOIL  AS 
INDICATED  BY  SEVERAL  DIFFERENT   METHODS 


Conditions 

Percentage  of 

KaO 

CaO 

MgO 

P306 

Bulk  analysis 

Productive  silt  loam 

1.98 
1.85 

1.05 
.89 

.039 
.039 

p.p.m. 

57 
52 

1.48 
1.50 

1.43 

1.48 

.452 
.422 

p.p.m. 

127 

45 

2.66 
3.57 

2.46 
3.32 

.220 
.144 

p.p.m. 

40 
23 

.23 

Unproductive  silt  loam 

Concentrated  HC1  digestion 
Productive  silt  loam 

.21 
.22 

Unproductive  silt  loam 

One  per  cent,  citric  acid 

Productive  silt  loam 

.20 
.101 

Unproductive  silt  loam 

Water  extract 

Productive  silt  loam 

.072 

p.p.m. 

12 

Unproductive  silt  loam 

5 

A  comparison  of  the  figures  from  the  good  and  poor  soil 
seems  to  indicate  no  differences  large  enough  to  warrant  opin- 
ions regarding  their  relative  fertility,  except  in  the  case  of  the 
water  extracts.  These  latter  figures,  however,  are  seasonal 
averages  and  required  as  long  a  time  to  procure  as  was  neces- 
sary to  grow  a  crop.  Such  fertility  measurement  is  not  as 
practicable  as  actually  using  the  crop  as  an  indicator. 

172.  Resume. — The  conclusion  that  chemical  analyses  are 
of  but  little  direct  practical  value  as  a  guide  to  soil  prod- 
uctivity is  unavoidable.  In  spite  of  the  great  importance  of 
chemistry  in  research  and  teaching,  it  fails  to  indicate  either 
the  permanent  or  the  immediate  fertility  of  the  land.  No 
chemical  method  is  capable  of  showing  substantial  and  con- 
stant differences  between  soils  producing  within  20  per  cent. 


CHEMICAL  ANALYSIS  OF  SOILS  327 

of  each  other.  Even  if  an  analysis  should  show  the  nutrients, 
which  would  be  available  over  a  term  of  years,  it  would  still  be 
inadequate,  since  available  nutrients  are  only  one  of  a  great 
number  of  factors  which  govern  productivity.1  This  produc- 
tivity equation  may  be  indicated  as  follows : 

Productivity  =  Texture  X  structure  X  organic  matter  X 
moisture  X  available  nutrients  X  soil  reaction  X  weather  X 
plant  disease  X  care  of  farmer,  etc.,  etc. 

The  factors  of  this  equation  are  variables,  their  importance 
in  determining  productivity  depending  on  many  things.  An 
accurate  knowledge  of  the  available  soil  nutrients,  even  if 
procurable,  would  aid  but  little  in  solving  such  an  equation. 

The  solution  of  individual  or  community  fertility  problems 
is  best  accomplished  by  the  aid  of  experienced  and  technically 
trained  men,  who  understand  the  scientific  principles  under- 
lying the  common  field  procedures  and  who  also  are  in  touch 
with  the  experiences  of  farmers  over  wide  and  diverse  areas. 
Such  men  may  advise  not  only  in  regard  to  the  crops  that 
should  be  grown  but  also  as  to  their  rotation,  management, 
and  fertilization  from  seeding  until  harvest.  These  men  may 
also  institute  such  cooperative  experiments  and  tests  as  will 
best  throw  light  on  fertility  problems  untouched  by  practical 
experience. 

1  The  samples  sent  to  a  chemical  laboratory  by  farmers  are  gen- 
erally improperly  taken  and  consequently  are  not  representative.  It 
would  be  unwise  to  analyze  such  soils  even  if  the  methods  were  capable 
of  showing  all  that  could  be  wished  for. 


CHAPTER  XVII 
'ALKALI  SOILS * 

It  has  already  been  shown  that  soils  are  acted  on  by  a  great 
variety  of  weathering  agents  which  gradually  render  soluble 
a  portion  of  the  most  susceptible  constituents.  This  soluble 
material  becomes  a  part  of  the  soil  solution  and  may  come  in 
contact  with  the  roots  of  any  crop  growing  on  the  land.  In 
humid  regions,  where  a  large  quantity  of  water  percolates 
through  the  soil,  this  soluble  matter  has  little  opportunity  to 
accumulate.2  In  arid  regions,  however,  where  loss  by  drainage 
is  slight,  these  salts  may  often  collect  in  large  amounts.  Dur- 
ing periods  of  dry  weather  they  are  carried  upward  by  the 
capillary  rise  of  the  soil-water,  while  during  periods  of  rain- 
fall they  may  move  downward  again  in  proportion  to  the  leach- 
ing action.  At  one  time  the  lower  soil  may  contain  consid- 
erably more  soluble  salt  than  the  upper ;  at  another  time  the 
condition  may  be  reversed,  in  which  case  the  solution  in  con- 
tact with  roots  may  contain  so  much  soluble  matter  that  vege- 
tation is  injured  or  destroyed.  This  excess  of  soluble  salts 
usually  has  a  marked  alkaline  reaction,  but  in  any  case  it  pro- 
duces what  is  termed  an  alkali  soil. 

Large  areas  of  land  in  every  continent  carry  soluble  salts 
to  such  an  extent  that  alkali  injury  is  either  actual  or  poten- 

1For  a  complete  and  satisfactory  treatise  on  alkali  see  Harris,  F.  S., 
Soil  Alkali,  New  York,  1920. 

^  3  Peat  soils  in  humid  regions  may  sometimes  contain  high  concentra- 
tions of  salts,  commonly  non-toxic,  and  lower  concentrations  of  ex- 
tremely toxic   salts. 

Conner,  S.  D.,  Excess  Soluble  Salts  in  Humid  Soils;  Jour.  Amer.  Soc. 
Agron.,  Vol.  9,  No.  6,  pp.  297-301,  1917. 

328 


ALKALI  SOILS  329 

tial.  It  is  estimated  that  13  per  cent,  of  the  irrigated  land  of 
the  United  States  contains  sufficient  soluble  salts  seriously  to 
interfere  with  crop  growth.  This  alone  amounts  to  nine  mil- 
lion acres  and  does  not  include  the  millions  of  acres  not  under 
the  ditch  that  are  affected  to  a  marked  degree  by  alkali.  Sim- 
ilar figures  are  available  from  other  continents  and,  since 
alkali  conditions  can  be  alleviated  and  controlled  to  a  certain 
extent,  the  importance  of  the  subject  becomes  apparent. 

Entirely  aside  from  the  economic  aspects,  alkali  is  of  great 
interest  scientifically,  offering  a  research  field  of  such  range 
and  complexity  as  to  involve  many  sciences.  A  greater  por- 
tion of  the  practical  information  regarding  alkali  and  its  con- 
trol has  arisen  from  the  purely  scientific  interest  that  has 
been  directed  towards  this  peculiar  soil  condition. 

173.  Composition  of  alkali. — It  has  been  emphasized  pre- 
viously that  the  solution  of  a  normal  humid-region  soil  is  of 
such  dilution  as  to  be  largely  ionic  in  character  except  in 
periods  of  low  moisture  content.  In  a  soil  affected  with 
alkali  it  is  obvious  that  the  molecular  state  is  dominant  and 
that  certain  salts  may  exist  and  function  as  definite  entities. 
Thus  the  following  bases  may  be  expected  to  be  present — 
sodium,  potassium,  magnesium,  calcium,  and  sometimes  am- 
monium. The  common  acid  radicals  are  chlorides,  sulphates, 
carbonates,  bicarbonates,  phosphates,  and  nitrates.  The  salts 
that  are  present  and  their  proportion  not  only  in  the  soil  solu- 
tion but  as  a  precipitant  will  vary  with  conditions. 

The  following  table  indicates  not  only  the  salts  that  may  be 
present  but  the  composition  of  the  alkali  as  reported  by  a 
number  of  different  investigators.    (See  table  LXXV,  p.  330.) 

174.  White  and  black  alkali. — Sulfates  and  chlorides  of 
the  alkalies,  when  concentrated  on  the  surface  of  the  soil, 
produce  a  white  incrustation,  which  is  very  common  in  alkali 
regions  during  a  dry  period  as  a  result  of  the  evaporation  of 
moisture.  Incrustations  of  this  character  are  called  white 
alkali. 


330 


NATURE  AND  PROPERTIES  OF  SOILS 


Table  LXXV 

COMPARISON   OF  ALKALI  EXPRESSED  IN  PERCENTAGE  OF  THE  DIF- 
FERENT  SALTS  PRESENT. 


1® 

o  x 

OH 

California 
(Tulare) 
Exp.  Sta.* 

Yakima,* 
Wash. 
12-24 
Inches 

Pillings,  Mont.' 

Yuma,  Ariz.* 

Salt 

Crust 

Surface 

10 
Inches 

Crust 

0-72 
Inches 

KCl 

K2S04 

K2C03 

Na2S04.... 

NaN03 

Na2C03.... 

NaCl 

Na2HP04.., 

MgS04 

MgCl2 

CaCl2 

NaHC03... 

CaS04 

Ca(HC03)2 
Mg(HC03)2 
(NH4)2C03 

1.6 

33.1 
6.6 

12.7 
17.3 

21.5 

3.9 

25.3 
19.8 
32.6 
14.7 
2.2 

1.4 

5.6 

9.7 

13.8 

36.7 

1.9 

16.5 

15.7 

1.6 
85.6 

.5 

8.9 

.6 

2.7 

21.4 

35.1 

7.3 

4.0 

22.0 
10.0 

4.0 

81.1 

7.7 
.2 
.3 

6.6 

22.0 

13.7 

6.9 
4.0 

21.0 
32.2 

Carbonates  of  the  alkalies,  particularly  sodium  carbonate, 
dissolve  organic  matter  from  the  soil,  thus  giving  a  dark  color 
to  the  solution  and  to  the  incrustation.  For  this  reason,  alkali 
containing  large  quantities  of  these  salts  is  called  black  alkali. 
Black  or  brown  alkali  may  also  be  produced  by  calcium  chlo- 
ride or  by  an  excess  of  sodium  nitrate. 

Black  alkali  is  much  more  destructive  to  vegetation  than  is 
the  white.    A  quantity  of  the  latter  which  would  not  seriously 

aHeadden,  W.  P.,  The  Fixation  of  Nitrogen;  Colo.  Agr.  Exp.  Sta., 
Bui.  155,  p.  10,  1910. 

2  Hilgard,  E.  W.,  Soils,  p.  442,  New  York,  1906. 

•Dorsey,  C.  W.,  Alkali  Soils  of  the  United  States;  U.  S.  Dept.  Agr., 
Bur.  Soils,  Bui.  35,  1906. 


ALKALI  SOILS  331 

interfere  with  the  growth  of  most  crops  might  completely  pre- 
vent the  development  of  useful  plants  if  the  alkali  were  black. 

175.  Origin  of  alkali. — While  the  presence  of  alkali  and 
its  influence  on  plants  has  been  known  for  centuries,  it  is 
only  within  recent  years  that  its  probable  mode  of  origin  has 
been  understood.  The  soluble  salts  have  undoubtedly  come 
from  the  materials  which  have  formed  the  soils,  the  reactions 
being  as  complex  as  the  ordinary  transformations  which  take 
place  in  soil  formation. 

Some  soils  have  been  laid  down  as  deltas  in  arms  of  the 
ocean.  If  these  bodies  of  water  later  are  cut  off  from  the  sea 
and  gradually  dry  up  under  arid  conditions,  an  alkali  soil 
will  be  left.  In  a  similar  way  saline  lakes  may  disappear  and 
soils  heavily  charged  with  alkali  will  result. 

The  commonest  mode  of  origin  for  alkali  soil  is  through 
ordinary  weathering  under  conditions  of  aridity.  Almost  any 
rock  will  give  rise  to  soils  rich  in  alkali  salts  if  leaching  is  not 
a  feature  in  the  weathering  processes.  In  western  United 
States  the  origin  of  much  of  the  soil  affected  to  the  greatest 
degree  with  alkali  is  associated  with  strata  originally  carrying 
much  soluble  material.  When  such  rock  forms  soil,  the  alkali 
arises  not  only  from  the  decomposition  of  the  minerals  of 
which  the  rock  is  composed,  but  is  greatly  reinforced  by  the 
soluble  salts  already  present.  The  Cretaceous  and  Tertiary 
beds  in  Utah,  Colorado,  and  Wyoming  are  of  this  character, 
having  been  laid  down  in  brackish  water.  They  naturally 
give  rise  to  soils  high  in  alkali.1 

One  fact  that  is  often  overlooked  in  practice  is  that  the 
amount  of  alkali  in  the  surface  layers  of  soil  may  be  greatly  in- 
creased by  improper  handling.  Rapid  evaporation  after  rain 
or  irrigation  will  carry  the  soluble  salts  toward  the  surface  and 
deposit  them  near  to  or  in  the  root  zone.    Again,  over-irriga- 

1  Stewart,  E.,  and  Peterson,  W.,  Origin  of  Alkali;  Jour.  Agr.  Res., 
Vol.  X,  No.  7,  pp.  331-353,  1917.  See  also,  Breazeale,  J.  F.,  Forma- 
tion of  Black  Alkali  in  Calcareous  Soils;  Jour.  Agr.  Res.,  Vol.  X,  No. 
11,  pp.  541-589,  1917. 


332        NATURE  AND  PROPERTIES  OF  SOILS 

tion  may  produce  leaching  into  lower  lands,  an  alkali  condition 
generally  resulting  if  the  areas  so  affected  remain  water-logged 
for  a  long  time. 

Very  often  alkali  is  localized  in  small  areas  called  alkali 
spots.  These  vary  in  size  from  a  few  square  yards  to  several 
acres.  In  years  of  good  rainfall  these  areas  may  be  pro- 
ductive, but  in  dry  years  they  are  often  quite  sterile.  Their 
origin  is  generally  due  to  seepage,  the  ground  water  being 
near  enough  the  surface  to  allow  a  concentration  of  salts  by 
capillarity,  especially  in  dry  seasons. 

A  very  peculiar  type  of  alkali  spot  occurs  in  the  Grand 
Valley  of  Colorado  and  elsewhere,  the  predominant  salt  being 
the  nitrate,  which  does  not  usually  occur  in  large  amounts 
as  alkali.  Two  theories  have  been  advanced  to  account  for  the 
presence  of  the  nitrate  salts.  One  hypothesis1  is  that  the 
surrounding  shales  are  comparatively  rich  in  nitrates  and  that 
the  alkali  accumulation  is  a  leaching  and  seepage  process.  The 
other  theory  is  biological  in  nature.2  Such  soils  are  capable 
of  rapid  nitrogen  fixation  by  means  of  their  bacterial  flora. 
The  idea  is  advanced  that  the  nitrogen  is  fixed  from  the  air 
very  rapidly  in  these  spots  and  later  oxidized  to  the  nitrate 
form.  Whatever  the  origin  of  the  soluble  salts  the  fact  re- 
mains that  such  spots  are  quite  destructive,  spreading  very 
rapidly  until  whole  orchards  are  wiped  out. 

Water  used  for  irrigation  is  very  often  heavily  charged  with 
alkali,  especially  where  any  amount  of  the  water  previously 
applied  to  the  soil  finds  its  way  back  into  the  streams.  At 
Canon  City,  Colorado,  the  Arkansas  River  is  very  pure.  At 
a  point  120  miles  below  the  soluble  salts  have  been  known 

1  Stewart,  E.,  and  Peterson,  W.,  The  Nitric  Nitrogen  Content  of  the 
Country  Bock;  Utah  Agr.  Exp.  Sta.,  Bui.  134,  1914. 

Also,  Further  Studies  of  the  Nitric  Nitrogen  Content  of  the  Country 
Bock;  Utah  Agr.  Exp.  Sta.,  Bui.  150,  1917. 

a  Headden,  W.  P.,  The  Fixation  of  Nitrogen  in  Colorado  Soils;  Colo. 
Agr.  Exp.  Sta.,  Bui.  186,  1913. 

Sackett,  W.  G.,  and  Isham,  E.  M.,  Origin  of  the  Niter  Spots  in 
Certain  Western  Soils;  Science,  N.  S.,  Vol.  42,  pp.  452-453,  1915. 


ALKALI  SOILS 


333 


to  reach  a  concentration  of  2200  parts  per  million.  The  quan- 
tity of  soluble  salts  that  may  be  present  in  irrigation  water 
before  it  is  unfit  for  use  depends  on  certain  conditions.  This 
amount  will  vary  with  the  crop,  the  rainfall,  the  soil,  the 
composition  of  the  alkali,  and  a  number  of  other  factors. 

i 

8 


I1 

i 


D£f>r/-f  or  *5o/l  //v  rte&T. 

Fig.  54. — Diagram  showing  the  amount  and  composition  of  alkali  salts 
at  various  depths  in  a  soil  at  Tulare,  California.     (After  Hilgard.) 


.ou 

<45 

A 

.35 

\ 

\ 

| 

.Jo 
.25 

\ 

.20 
./5 

./o 

.05 

1 AU 

1  RA/\ 

\ 

rs* 

V 

CS 

'-1 

_     -- 

-^ 

i 
.J 

?e__\ 

o 

/ 

t 

> 

J 

\ 

4- 

Where  the  alkali  is  of  the  sodium  sulfate  type  rather  high 
concentrations  are  admissible,  running  as  high  as  1000  parts 
per  million.  Water  carrying  black  alkali  must  be  used  with 
great  caution.  Table  LXXVI  indicates  the  concentration  that 
may  be  expected  in  normal  irrigation  water. 

The  preponderance  of  sodium  chloride  is  almost  always  a 
feature,  not  only  in  alkali  water  but  also  in  soils  affected  with 
alkali  salts.    This  may  be  explained  as  due  to  differential  ab- 


334        NATURE  AND  PROPERTIES  OF  SOILS 
Table  LXXVI 

ANALYSIS  OF  SOME  TYPICAL  ALKALINE  RIVER  WATER  OF  WESTERN 
UNITED  STATES.1 


Stream 

Total 
Solids 
p.p.m. 

Percentage  of  Total  Solids  as 

CI 

S04 

CO, 

Na 

K 

Ca 

Mg 

SiOa 

Malad  River,  Utah  .... 

Sevier    River    at   Delta, 

Utah   

4,395 

1,316 
791 

3,747 
936 

1,972 

50.0 

25.0 
21.6 
7.4 
13.8 
39.9 

2.9 

24.1 
30.1 
17.3 
29.0 
7.3 

4.7 

17.9 
11.5 
35.1 
38.3 
9.6 

37.4 

16.4 
14.8 
23.5 
13.1 
24.9 

".8 

1.4 

5.4 

.6 

5.3 

13.7 

10.1 

6.6 

6.6 

6.5 
3.0 
2.2 
7.7 
2.9 

Rio  Grande,  Texas  .... 
Mill  Creek,  Montana  . . 
San  Benito,  California. 
Buckeye  Canal,  Arizona 

3.8 

.7 

2.6 

2.7 

sorption  of  ions  by  the  soil.  Sodium  and  chlorine  ions  seem 
to  be  about  as  little  absorbed  by  the  soil  as  any  of  the  com- 
mon soil  constituents.  They  are  thus  readily  carried  through 
the  soil  and  are  free  to  accumulate  in  considerable  amounts 
at  points  where  they  may  become  noticeable.  Their  union  of 
necessity  produces  large  quantities  of  sodium  chloride  or  com- 
mon salt.2 

176.  Effect  of  alkali  on  crops. — The  presence  of  rela- 
tively large  amounts  of  salts  dissolved  in  water  and  brought 
into  contact  with  a  plant  cell  has  been  shown  to  cause  a  shrink- 
age of  the  protoplasmic  lining  of  the  cell.  This  action,  called 
plasmolysis,  increases  with  the  concentration  of  the  solution 
until  the  plant  finally  dies.  The  phenomenon  is  due  to  the 
osmotic  movement  of  the  water,  which  passes  from  the  cell 
towards  the  more  concentrated  soil  solution.  The  nature  of  the 
salt,  the  species,  and  even  the  individuality  of  the  plant,  as 
well  as  other  factors,  determine  the  exact  concentration  at 
which  the  plant  succumbs.  The  carbonates  of  the  alkali  bases 
have,  in  addition,  a  corroding  effect  on  the  plant  tissues,  dis- 

1  Harris,  F.  S.,  Soil  Alkali,  p.  232;  New  York,  1920. 
aDorsey,  C.  W.,  Alkali  Soils  of  the  United  States;  U.  S.  Dept.  Agr., 
Bur.  Soils,  Bui.  35,  1906. 


ALKALI  SOILS 


335 


solving  the  parts  of  the  plant  with  which  they  come  into  con- 
tact. Such  action  is  not  as  important  as  plasmolysis  and  when 
it  does  occur  is  most  noticeable  at  the  root  crown.  (See 
Fig.  55.) 

Indirectly,  alkali  may  influence  plants  by  its  effect  on  soil 
tilth,  soil  organisms,  and  fungous  and  bacterial  growths.  Mar- 
chal,1  for  example,  found  that  the  formation  of  nodules,  con- 
taining the  nitrogen-fixing  organisms,  did  not  develop  well 


Fig.  55. — )(1)    Cross-section  diagram  of  a  normal  plant  cell, 
after  plasmolysis  has  taken  place. 


(2)    Cell 


on  pea  roots  in  nutrient  solutions  when  certain  concentra- 
tions of  salts  were  maintained.  Ammonium  salts  were  injuri- 
ous at  a  concentration  of  500  parts  per  million.  Potassium  and 
sodium  salts  retarded  the  nodule  development  at  5000  and  3333 
parts  to  the  million  respectively.  The  quantity  of  alkali  that 
will  cause  injury  to  ammonifying  and  nitrifying  bacteria 
varies  from  250  to  4000  parts  per  million,  depending  on  con- 
ditions. 

177.  Resistance  of  different  plants  to  alkali. — The  fac- 
tors that  determine  the  tolerance  of  plants  toward  alkali  are : 

1  Marchal,  E.,  Influence  des  Sels  mineraux  nutritifs  sur  la  Production 
des  nodosites  chez  le  Pois;  Compt.  Bend.  Acad.  Sci.  (Paris),  Tome  133, 
No.  24,  p.  1032,  1901. 


336        NATUKE  AND  PROPERTIES  OF  SOILS 

(1)  the  physiological  constitution  of  the  plant,  and  (2)  the 
rooting  habit.  The  former  is  little  understood,  so  much  de- 
pending on  the  character  of  the  alkali  solution,  the  nature 
of  the  cell-wall,  and  the  character  and  activity  of  the  cell  con- 
tents. It  has  long  been  known  that  the  toxicity  of  two  salts 
when  together  is  considerably  less  than  the  sum  of  their  detri- 
mental action  when  used  alone.  This  ameliorating  or  antagon- 
istic action  varies  for  different  salts,  seeming  to  be  greatest 
when  calcium  and  magnesium  are  involved.  This  is  but  an 
example  of  the  complexities  which  arise  when  an  attempt  is 
made  to  study  the  physiological  relationships  of  alkali  injury. 

The  rooting  habit  of  plants  in  their  relation  to  alkali  toler- 
ance is  more  easily  understood.  The  advantage  is  always  with 
deep-rooted  crops,  such  as  alfalfa  and  sugar-beets,  probably 
because  a  portion  of  the  root  may  be  in  a  less  strongly  impreg- 
nated part  of  the  soil. 

The  tolerance  of  many  plants  to  alkali  has  been  studied  in 
water  culture.  Such  results  are  not  of  great  practical  value, 
however,  as  it  is  only  in  soil  that  all  of  the  numerous  factors, 
such  as  absorption,  antagonism,  and  physical  conditions,  come 
into  play.  Harris  and  Pittman  *  found  that  organic  matter 
in  a  soil  had  a  marked  ameliorating  influence  on  alkali  injury, 
especially  from  sodium  carbonate.  High  moisture  was  also  an 
important  factor  in  lowering  the  toxicity  of  soluble  salts. 

Guthrie  and  Helms,2  using  a  rich  garden  loam,  found  the  fol- 
lowing concentrations  slightly  affecting  or  entirely  preventing 
germination  and  growth  of  certain  crops.    (Table  LXXVII.) 

Of  the  cereals,  barley  and  oats  are  the  most  tolerant,  these 
being  able,  in  some  cases,  to  produce  good  crops  in  soil  con- 
taining two-tenths  per  cent,  of  white  alkali.  Of  the  forage 
crops,  a  number  of  valuable  grasses  are  able  to  grow  on  soil 

1  Harris,  F.  S.,  and  Pittman,  D.  W\,  Soil  Factors  Affecting  the 
Toxicity  of  Alkali;  Jour.  Agr.  Ees.,  Vol.  XV,  pp.  287-319,  1918. 

3  Guthrie,  F.  B.,  and  Helms,  R.,  Pot  Experiments  to  Determine  the 
Limits  of  Endurance  of  Different  Farm  Crops  for  Certain  Injurious  Sub- 
stances; Agr.  Gaz.,  N.  S.  Wales,  Vol.  14,  No.  2,  pp.  114-120,  1903. 


ALKALI  SOILS 


337 


Table  LXXVII 

EFFECT  OF  CERTAIN  CONCENTRATIONS  OF  SALTS  ON  CROPS.   EX- 
PRESSED IN  PARTS  PER  MILLION. 


Condition 

NaCl 

Na2C0, 

Wheat 

Barley 

Rye 

Wheat 

Barley 

Rye 

Germination  affected. 
Germination  prevented 

Growth  affected 

Growth  prevented.  . .  . 

500 
2000 

500 
2000 

1000 
2500 
1000 
2000 

1000 
4000 
1500 
2000 

3000 
5000 
1000 
4000 

2500 
6000 
1500 
4000 

2500 
5000 
2500 
4000 

containing  considerably  more  than  two-tenths  per  cent  of  al- 
kali. Timothy,  smooth  brome,  and  alfalfa  are  the  cultivated 
forage  plants  most  tolerant  of  alkali,  although  they  do  not 
equal  the  native  grasses  in  this  respect.  Cotton  also  tolerates 
a  considerable  amount  of  alkali. 

Loughridge,1  after  experiments  and  observation  for  a  num- 
ber of  years,  has  obtained  data  regarding  the  resistance  of 
various  crops  to  the  several  alkali  salts.  His  results  are  given 
in  part  as  follows,  expressed  in  pounds  to  an  acre  to  a  depth  of 
four  feet.    ( See  table  LXXVIII,  page  338. ) 

Although  in  general  the  results  as  to  the  resistance  to  alkali 
of  the  various  crops  are  so  conflicting,  the  Bureau  of  Soils,2 
in  its  alkali  mapping,  has  been  able  to  make  a  rough  classifi- 
cation as  follows.     (See  table  LXXIX,  page  338.) 

178.  Conditions  that  influence  the  effect  of  alkali. — It 
has  already  been  mentioned  that  organic  matter  and  a  high 
moisture  content  of  the  soil  tended  to  alleviate  alkali  toxicity. 
Should,  however,  a  previously  wet  soil  become  dry,  the  solu- 
tion, originally  very  dilute,  would  become  concentrated  and 

1  Loughridge,  R.  H.,  Tolerance  of  Alkali  by  Various  Cultures;  Calif. 
Agr.  Exp.  Sta.,  Bui.  133,  1901.  See  also  Kearney,  T.  H.,  and  Harter, 
L.  L.,  Comparative  Tolerance  of  Various  Plants  for  the  Salts  Com- 
mon in  Alkali  Soils;  U.  S.  Dept.  Agr.,  Bur.  Plant  Ind.,  Bui.  113,  1907. 

"Dorsey,  C.  W.,  Alkali  Soils  of  the  United  States;  U.  S.  Dept.  Agr., 
Bur.  Soils,  Bui.  35,  pp.  23-25,  1906. 


338 


NATURE  AND  PROPERTIES  OF  SOILS 


Table  LXXVIII 


Crop 


Grapes. . . . 
Oranges .  . . 

Pears 

Apples.  . . . 
Peaches. . . 

Rye 

Barley .... 
Sugar  Beet 
Sorghum . . 
Alfalfa.... 
Saltbush .  . . 


Na*S04 


40,800 

18,600 

17,800 

14,240 

9,600 

9,800 

12,020 

52,640 

61,840 

102,480 

125,640 


Na,CO, 


7,550 

3,840 

1,760 

640 

680 

960 

12,170 

4,000 

9,840 

2,360 

18,560 


NaCl 


9,640 
3,360 
1,360 
1,240 
1,000 
1,720 
5,100 
5,440 
9,680 
5,760 
12,520 


Total 
Alkali 


45,760 
21,840 
20,920 
16,120 
11,280 
12,480 
25,520 
59,840 
8.1,360 
110,320 
156,720 


consequently   toxic.      High    moisture   should,    therefore,    be 
maintained  at  least  as  long  as  the  crop  is  upon  the  soil. 

The  distribution  of  the  alkali  at  different  depths  may  have 
an  important  bearing  as  to  its  effect  on  plants.  Young  plants 
and  shallow-rooted  crops  may  be  entirely  destroyed  by  the 
concentration  of  alkali  at  the  surface,  while  the  same  quantity 
evenly  distributed  through  the  soil,  or  carried  by  moisture  to 
a  lower  depth,  would  have  caused  no  injury.    A  loam  soil,  by 

Table  LXXIX 


Percentage  of 

Total  Salts  in 

Soil 


.00—  .20 
.20—  .40 
.40—  .60 

.6O—1.00 
1.00—3.00 


Percentage  of 

Black  Alkali 

in  Soil 


Crops 


All  crops  grow 
All  but  most  sensitive 
Old  alfalfa,  sugar  beet,  sorghum, 
barley 

Only  most  resistant  plants 
No  plants 


ALKALI  SOILS  339 

reason  of  its  greater  water-holding  capacity  and  absorptive 
power,  will  contain  more  alkali  without  injury  to  plants  than 
will  a  sandy  soil.  Certain  of  the  alkali  salts  exert  a  deflocculat- 
ing  action  on  clay  soils  and  effect  an  indirect  injury  in  that 
way. 

In  irrigated  regions  the  injurious  effects  of  alkali  are  in 
many  cases  developed  only  after  irrigation  has  Been  practiced 
for  a  few  years.  This  is  due  to  what  is  known  as  a  "rise  of 
alkali"  and  comes  about  through  the  accumulation,  near  the 
surface  of  the  soil,  of  salts  that  were  formerly  distributed 
throughout  a  depth  of  perhaps  many  feet.  Before  the  land 
was  irrigated  the  rainfall  penetrated  only  a  slight  depth  into 
the  soil,  and  when  evaporation  took  place  salts  were  drawn  to 
the  surface  from  only  a  small  volume  of  soil.  When,  however, 
irrigation  water  is  turned  on  the  land,  the  soil  becomes  wet  to 
a  depth  of  perhaps  fifteen  or  twenty  feet.  During  the  por- 
tion of  the  year  in  which  the  soil  is  allowed  to  dry  large  quan- 
tities of  salts  are  carried  toward  the  surface  by  the  upward- 
moving  capillary  water. 

Although  these  salts  are  in  part  carried  down  again  by  the 
next  irrigation  the  upward  movement  constantly  exceeds  the 
downward  one.  This  is  because  the  descending  water  passes 
largely  through  the  non-capillary  interstitial  spaces,  while  the 
ascending  water  passes  almost  entirely  through  the  capillary 
channels.  The  smaller  spaces,  therefore,  contain  a  consider- 
able quantity  of  soluble  salts  after  the  downward  movement 
ceases  and  the  upward  movement  begins.  In  other  words,  the 
volume  of  water  carrying  the  salts  downward  in  the  capil- 
lary spaces  is  less  than  that  carrying  them  upward  through 
these  spaces.  Surface  tension  causes  the  salts  to  accumulate 
largely  in  the  capillary  spaces,  and  it  is,  therefore,  the  direc- 
tion of  the  principal  movement  through  these  spaces  that  de- 
termines the  point  of  accumulation  of  the  alkali. 

There  are  large  areas  of  land  in  Egypt,  in  India,  and  even 
in  France  and  Italy,  as  well  as  in  this  country,  that  have  suf- 


340        NATURE  AND  PROPERTIES  OF  SOILS 

fered  in  this  way,  and  not  infrequently  they  have  reverted 
to  a  desert  state. 

179.  Alkali  vegetation. — There  are  a  great  number  of 
plants  that  seldom  grow  on  soils  other  than  those  affected  with 
alkali.  Davy  1  states  that  there  are  197  species  restricted  to 
alkali  soils  in  California.  Such  plants  are  generally  recog- 
nized by  the  farmers  in  the  district  as  indicators  of  alkali. 
Care  should  be  taken,  however,  in  thus  classifying  alkali  land. 
Such  plants  should  occupy  the  land  to  the  exclusion  of  less 
tolerant  species.  Some  of  the  plants 2  whose  presence  should 
cause  one  to  surmise  alkali  conditions  are  as  follows : 

Greasewood  Inkweed 

Alkali-heath  Tussock-grass 

Salt-grass  Bushy  samphire 

Salt-bush  Spike-weed 

Cressa  Rabbit  bush 

Sage-brush,  which  is  so  often  associated  in  popular  literature 
with  alkali,  does  not  grow  on  land  which  carries  a  great  amount 
of  soluble  salts.  In  locating  land  it  is,  therefore,  a  good  indi- 
cator of  alkali-free  conditions,  especially  if  it  is  growing  vig- 
orously. 

180.  The  handling  of  alkali  lands.3 — Ordinarily  there 
are  two  general  ways  in  which  alkali  lands  may  be  handled  in 
order  to  avoid  the  injurious  effects  of  soluble  salts.  The  first 
of  these  is  eradication,  the  second  may  be  designated  as  con- 
trol.   In  the  former  case  an  attempt  is  made  actually  to  elimi- 

1  Davy,  J.  B.,  Investigations  on  the  Native  Vegetation  of  Alkali 
Lands;  Calif.  Exp.  Sta.  Kep.,  1895-97,  pp.  53-75. 

2  Harris,  F.  S.,  Soil  Alkali;  Chap.  VI,*  New  York,  1920. 

"Dorsey,  C.  W.,  Reclamation  of  Alkali  Soils;  U.  S.  Dept.  Agr.,  Bur. 
Soils,  Bui.  34,  1906.  Also,  Hilgard,  E.  W.,  Utilization  and  Reclamation 
of  Alkali  Soils;  New  York,  1911.  Also,  Brown,  C.  F.,  and  Hart,  E.  A., 
Reclamation  of  Seeped  and  Alkali  Lands;  Utah  Agr.  Exp.  Sta.,  Bui.  Ill, 
1910.  Also,  Dorsey,  C.  W.,  Reclamation  of  Alkali  Soils  at  Billings, 
Montana;  U.  S.  Dept.  Agr.,  Bur.  Soils,  Bui.  44,  1907.  Also  Harris, 
F.  S.,  Soil  Alkali;  Chaps.  XII,  XIII  and  XIV;  New  York,  1920. 


ALKALI  SOILS  341 

nate  by  various  means  some  of  the  alkali.  In  the  latter,  meth- 
ods of  soil  management  are  employed  which  will  keep  the  salts 
well  distributed  throughout  the  soil.  In  many  cases  soils 
would  grow  excellent  crops  if  the  alkali  could  only  be  kept 
well  distributed  through  the  soil  layers  so  that  no  concentra- 
tion that  is  toxic  could  occur,  at  least  within  the  root  zone. 
In  general,  steps  should  be  taken  toward  the  control  of  alkali, 
whether  eradication  is  attempted  or  not.  Under  irrigation, 
careful  attention  is  always  wise. 

181.  Eradication  of  alkali. — Of  methods  designed  at 
least  partially  to  free  the  soil  of  alkali  the  commonest  are: 
(1)  leaching  with  under-drainage,  (2)  correction  with  gyp- 
sum, (3)  scraping,  and  (4)  flushing.  Of  the  various  methods 
for  removing  an  excess  of  soluble  salts,  the  use  of  tile  drains 
is  the  most  thorough  and  satisfactory.  When  this  method  is 
used  in  an  irrigated  region  heavy  and  repeated  applications 
of  water  must  be  made,  to  leach  out  the  alkali  from  the  soil 
and  drain  it  off  through  the  tile.  When  used  for  the  ameliora- 
tion of  alkali  spots  in  a  semi-arid  region,  the  natural  rainfall 
will  often  in  time  effect  the  removal. 

In  laying  tiles  it  is  necessary  to  have  them  at  such  a  depth 
that  the  soluble  salts  in  the  soil  beneath  them  will  not  readily 
rise  to  the  surface.  This  will  depend  on  those  properties  of  the 
soil  governing  the  capillary  movement  of  water.  Three  or 
four  feet  in  depth  is  usually  sufficient,  but  the  capillary  move- 
ment should  first  be  estimated. 

After  the  drains  have  been  placed  the  land  is  flooded  with 
water  to  a  depth  of  several  inches.  The  water  is  allowed  to 
soak  into  the  soil  and  to  pass  off  through  the  drains,  leaching 
out  part  of  the  alkali  in  the  process.  Before  the  soil  has 
time  to  become  very  dry  the  flooding  is  repeated,  and  the 
operation  is  kept  up  until  the  land  is  brought  into  a  satis- 
factory condition. 

Crops  that  will  stand  flooding  may  be  grown  during  this 
treatment,  and  they  will  serve  to  keep  the  soil  from  puddling, 


342        NATURE  AND  PROPERTIES  OF  SOILS 

as  it  is  likely  to  do  if  allowed  to  become  dry  at  the  surface. 
If  crops  are  not  grown,  the  soil  should  be  harrowed  between 
floodings.  The  operation  should  not  be  carried  to  a  point 
where  the  soluble  salts  are  reduced  below  the  needs  of  the 
crop.1  The  use  of  gypsum  on  black  alkali  land  has  sometimes 
been  practiced  for  the  purpose  of  converting  the  alkali  carbon- 
ates into  sulfates,  thus  ameliorating  the  injurious  properties 
of  the  alkali  without  decreasing  the  amount.  The  quantity 
of  gypsum  required  may  be  estimated  from  the  amount  and 
composition  of  the  alkali.  The  soil  must  be  kept  moist,  in 
order  to  bring  about  the  reaction,  and  the  gypsum  should  be 
harrowed  into  the  surface,  not  plowed  under.  The  reaction 
is  as  follows : 

Na2C03  +  CaS04  =  CaC03  +  Na2S04 

When  soil  containing  black  alkali  is  to  be  tile-drained,  it 
is  recommended  that  the  land  should  first  be  treated  with  gyp- 
sum, as  the  substitution  of  alkali  sulfates  or  carbonates  causes 
the  soil  to  assume  a  much  less  compact  condition  and  thus  fa- 
cilitates drainage.  It  also  prevents  the  loss  of  organic  matter 
dissolved  by  the  carbonate  of  soda  and  the  soluble  phosphates, 
both  of  which  are  precipitated  by  the  change. 

Removal  of  the  alkali  incrustation  that  has  accumulated  at 
the  surface  is  sometimes  resorted  to.  Very  often  the  rise  of 
alkali  is  encouraged  by  applications  of  irrigation  water,  which 
is  allowed  to  evaporate  unretarded.  The  salts  are  thus  carried 
upward  by  the  capillary  movement  of  the  soil-water.     This 

1  It  has  been  suggested  that  elemental  sulfur  could  be  used  to  advan- 
tage in  alkali  land,  especially  where  carbonates  and  bicarbonates  abound. 
Sulfur  generally  oxidizes  in  the  soil  quite  readily,  producing  an  acid 
[see  par.  221].  Instead  of  trying  to  remove  all  of  the  alkalinity  by 
leaching,  it  might  be  more  practicable  to  add  sulfur. 

Lipman,  J.  G.,  Sulfur  on  Alkali  Lands;  Soil  Sci.,  Vol.  II,  No.  3, 
p.  205,  1916. 

Hibbard,  P.  L.,  Sulfur  for  Neutralizing  Alkali  Soil;  Soil  Sci.,  Vol, 
XI,  No.  5,  pp.  385-387,  1921. 


ALKALI  SOILS  343 

method  of  alkali  eradication  is  never  very  efficient,  and  is  often 
dangerous,  as  it  encourages  the  presence  of  very  large  amounts 
of  alkali  salts  in  the  surface  soil. 

Often  alkali  accumulations  may  be  washed  from  the  soil  sur- 
face by  turning  on  a  rapidly  moving  stream  of  water.  The  tex- 
ture of  the  soil,  as  well  as  the  slope  of  the  land,  must  be  just 
right  for  such  a  procedure.  Generally  so  much  water  enters 
the  soil  that  the  land  remains  heavily  impregnated  with  alkali 
salts.  Both  this  method  and  the  previous,  even  if  successful, 
are  only  temporary.  Moreover,  lands  carrying  so  much  alkali 
as  to  admit  of  either  one  of  these  procedures  may  be  so  heavily 
charged  as  never  to  yield  to  any  form  of  either  eradication 
or  control. 

182.  Control  of  alkali. — Where  excessive  amounts  of 
soluble  salts  do  not  exist  in  a  soil  the  control  of  the  alkali  with 
a  view  of  keeping  it  well  distributed  in  the  soil  column  is  the 
best  practice.  The  retardation  of  evaporation  is,  of  course,  the 
main  object  in  this  procedure.  The  intensive  use  6f  the  soil- 
mulch  is,  therefore,  to  be  advocated,  especially  in  al]  irrigation 
operations  where  alkali  concentrations  are  likely  to  occur. 
Such  a  method  of  soil  management  not  only  saves  moisture,  but 
also  prevents  the  excessive  translocation  of  soluble  salts  into 
the  root  zone.  This  method  of  control  is  the  most  economical, 
the  cheapest,  and  the  one  to  be  advocated  on  all  occasions,  no 
matter  what  may  have  been  the  previous  means  of  dealing  with 
the  alkali  situation.  Certain  soils  that  are  strongly  impreg- 
nated with  alkali  may  be  gradually  improved  by  cropping  with 
sugar-beets  and  other  crops  that  are  tolerant  of  alkali  and 
that  remove  large  quantities  of  salts.  This  is  more  likely  to  be 
efficacious  where  irrigation  is  not  practiced.  Certain  crops, 
moreover,  while  somewhat  seriously  injured  when  young,  are 
very  resistant  once  their  root  systems  are  developed.  A  good 
example  is  alfalfa,  the  young  plants  being  very  tender  while 
the  mature  ones  are  extremely  resistant.    Temporary  eradica- 


344        NATURE  AND  PROPERTIES  OF  SOILS 

tion  of  alkali  may  allow  such  a  crop  to  be  established.  Farm 
manure  has  been  found  especially  useful  in  this  respect.1  The 
crop  once  well  established  will  then  maintain  itself  in  spite  of 
the  concentrations  that  may  occur  later. 

1  Lipman,  C.  B.,  and  Gericke,  W.  F.,  The  Inhibition  by  Stable  Manure 
of  the  Injurious  Effects  of  Alkali  Salts  in  Soils;  Soil  Sci.,  Vol.  VII, 
No.  2,  pp.  105-120,  1919. 


CHAPTER  XVIII 
SOIL  ACIDITY 

A  chemical  or  physico-chemical  viewpoint  regarding  the 
soil  and  its  solution  is  essential  in  explaining  many  of  the  phe- 
nomena, especially  those  relating  to  higher  plants  and  their 
nutrition.  Since  plants  respond  so  markedly  to  their  chemical 
environment,  the  importance  of  soil  reaction  has  long  at- 
tracted much  attention.  Two  conditions  are  popularly  recog 
nized  in  this  respect — soil  alkalinity  or  alkali  and  soil  acidity. 
The  former  condition  can  only  occur  where  soluble  salts  may 
concentrate  in  the  soil  and  is  confined  largely  to  arid  and  semi- 
arid  regions.  Soil  acidity,  on  the  other  hand,  is  common  only 
in  humid  sections.  So  widespread  is  it  occurrence  and  so 
marked  is  its  influence  on  crop  yields  that  its  importance  in 
a  practical  way  surpasses  that  of  soil  alkali. 

183.  General  nature  of  soil  acidity.1 — The  nature  of  soil 
acidity  is  so  little  understood  that  it  is  impossible  to  define 
or  explain  it  except  in  the  most  general  terms.  So-called  soil 
acidity  may  be  considered  for  practical  purposes  as  a  more 
or  less  unfavorable  condition  for  plant  growth,  arising  in  the 
soil  through  a  lack  of  certain  active  bases  such  as  calcium  and 
magnesium  and  which  in  practice  is  alleviated  by  the  addition 
of  some  form  of  lime.2 

Technically  three  reasons  may  be  suggested  as  accounting 
for  the  harmful  effects  of  soil  acidity:  (1)  unfavorable  hydro- 

xMacIntire,  W.  H.,  The  Nature  of  Soil  Acidity  with  Regard  to  its 
Quantitative  Determination;  Jour.  Amer.  Soc.  Agron.,  Vol.  13,  No.  4, 
pp.  137-161,  1921. 

aLime  in  an  agricultural  sense  refers  to  all  of  the  compounds  of  cal- 
cium and  magnesium  commonly  utilized  in  correcting  soil  acidity. 

345 


346        NATURE  AND  PROPERTIES  OF  SOILS 

gen  ion  concentrations;1  (2)  presence  of  substances  harm- 
ful to  plant  growth  such  as  active  aluminum,  manganese  and 
the  like,  the  presence  of  which  is  usually  accompanied  by  a 
hydrogen  ion  concentration  beyond  neutrality;  and  (3)  im- 
proper nutrition  arising  from  a  lack  of  calcium  as  a  nutrient 
or  as  a  synergistic  agent  in  facilitating  the  entrance  of  other 
nutrient  ions  into  the  plant.2 

184.  Hydrogen  ion  concentration. — A  number  of  condi- 
tions are  possible  if  the  toxic  influence  of  soil  acidity  is  due  to 
an  actual  acid.  The  harmful  effect  might  be  due  to  an  ab- 
normally high  hydrogen  ion  concentration  arising  from  (1) 
soluble  organic  or  inorganic  acids  in  the  soil  solution.  Again 
it  might  be  due  to  (2)  insoluble  acids  or  acid  salts  which,  on 
reaction  with  water,  produce  acidity.  In  this  case,  the  hydro- 
gen ion  concentration  of  the  soil  solution  at  any  particular  time 
would  not  be  a  measure  of  the  so-called  soil  acidity.8  A  harm- 
ful hydrogen  ion  influence  may  also  be  ascribed  (3)  to  soluble 
acids,  either  organic  or  mineral,  absorbed  by  the  soil  complexes 
and  which  would  become  active  only  under  certain  conditions. 
An  additional  feature  of  the  actual  acidity  theory  may  lie  in 
(4)  the  selective  absorption  of  bases  by  the  soil,  by  which  acid- 
ity might  be  developed  from  neutral  or  even  alkaline  salts. 
If  the  actual  acidity  explanation  is  entertained,  any  one  or  all 
of  these  phases  might  be  considered  as  contributing  to  the  dele- 
terious effects  so  noticeable  on  certain  plants. 

185.  Active  toxic  bases. — The  explanation  of  the  harm- 
ful effects  of  so-called  soil  acidity  as  being  due  to  the  presence 
of  active  toxic  bases  has  of  late  received  much  attention.    The 

1  Hydrogen  is  the  one  essential  constituent  of  all  acids.  When  dis- 
solved in  water,  acids  dissociate,  the  hydrogen  ion  becoming  active.  The 
strength  of  an  acid  is  determined  by  its  hydrogen  ion  concentration. 

2  True  speaks  of  this  cooperative  relationship  as  synergism.  By  it 
calcium  makes  other  nutrients  physiologically  available.  True,  E.  H., 
The  Function  of  Calcium  in  the  Nutrition  of  Seedlings;  Jour.  Amer. 
Soc.  Agron.,  Vol.  13,  No.  3,  pp.  91-107,  1921. 

8Eice,  F.  E.,  and  Osugi,  S.,  The  Inversion  of  Cane  Sugar  by  Soils 
and  Allied  Substances  and  the  Nature  of  Soil  Acidity ;  Soil  Sci.,  Vol.  V, 
No.  5,  p.  347,  1918. 


SOIL  ACIDITY  347 

presence  of  active  aluminum  in  so-called  acid  soils  has  been 
known  for  some  time.  Abbott,  Conner,  and  Smalley  1  showed 
in  1913  that  aluminum  salts  were  the  toxic  agents  in  a  certain 
unproductive  soil.  In  1918,  Hartwell  and  Pember  2  proved 
quite  definitely  that,  for  certain  soils  and  for  certain  crops, 
the  aluminum  ion  was  the  injurious  factor  rather  than  the 
hydrogen  ion  that  accompanied  it.  The  work  of  Mirasol 3  indi- 
cates that  active  aluminum  is  usually  present  in  acid  soils.4 

Although  soluble  iron  is  seldom  present  to  an  excess,  its 
ferrous  salts  are  known  to  be  toxic  to  a  greater  extent  than 
acids  of  the  same  concentration.5  While  soluble  iron  may  ac- 
company active  aluminum,  it  is  questionable  whether  it  ac- 
tually figures  in  acidity  effects.  The  toxic  influence  of  man- 
ganese is  more  probable,  since  it  is  more  soluble  in  an  acid  than 
a  neutral  soil.  While  it  is  extremely  toxic  to  plants  above 
a  certain  concentration  the  recent  work  of  Funchess 6  with 

1  Abbott,  J.  B.,  Conner,  S.  D.,  and  Smalley,  H.  E.,  Soil  Acidity,  Nitri- 
fication and  the  Toxicity  of  Soluble  Salts  of  Aluminum;  Ind.  Agr.  Exp. 
Sta.,  Bui.  170,  1913. 

2  Hartwell,  B.  L.,  and  Pember,  F.  R.,  The  Presence  of  Aluminum  as  a 
Beason  for  the  Difference  in  the  Effect  of  So-called  Acid  Soil  on  Barley 
and  Bye;  Soil  Sci.,  Vol.  VI,  No.  4,  pp.  259-277,  1918. 

8  Mirasol,  J.  J.,  Aluminum  as  a  Factor  in  Soil  Acidity;  Soil  Sci., 
Vol.  X,  No.  3,  pp.  153-193,  1920. 

4  See  also,  Kratzman,  E.,  Zur  Physiologischen  Wirkung  der  Aluminium 
Sals  auf  die  Pflanze;  Chem.  Ztg.,  Jahrgang  38,  S.  1040,  1914. 

Ruprecht,  R.  W.,  Toxic  Effect  of  Iron  and  Aluminum  Salts  on  Clover 
Seedlings;  Mass.  Agr.  Exp.  Sta.,  Bui.  161,  1915. 

Miyake,  K.,  The  Toxic  Action  of  Soluble  Aluminum  Salts  upon  the 
Growth  of  the  Bice  Plant;  Jour.  Biol.  Chem.,  Vol.  25,  No.  1,  pp. 
23-28,  1916.  M 

Conner,  S.  D.,  Liming  in  Its  Belation  to  Injurious  Inorganic  Com- 
pounds in  the  Soil;  Jour.  Amer.  Soc.  Agron.,  Vol.  13,  No.  3,  pp.  113- 
124,  1921. 

6  Conner,  S.  D.,  Liming  in  Its  Belation  to  Injurious  Inorganic  Com- 
pounds in  the  Soil;  Jour.  Amer.  Soc.  Agron.,  Vol.  13,  No.  3,  p.  114, 
1921. 

•Funchess,  M.  J.,  Acid  Soils  and  the  Toxicity  of  Manganese;  Soil 
Sci.,  Vol.  VIII,  No.  1,  p.  69,  1919. 

See  also,  Kelly,  W.  P.,  The  Influence  of  Manganese  on  the  Growth 
of  Pineapples;  Haw.  Agr.  Exp.  Sta.,  Bui.  23,  1909. 

Skinner,  J.  J.,  and  Reid,  F.  R.,  The  Action  of  Manganese  Under  Acid 
and  Neutral  Soil  Conditions;  U.  S.  Dept.  Agr.,  Bui.  441,  1916. 


348        NATURE  AND  PROPERTIES  OF  SOILS 

Alabama  soils  indicates  that  it  is  probably  of  minor  importance 
as  compared  with  aluminum.  A  toxic  effect  from  magnesium 
is  possible,  especially  if  there  is  not  enough  calcium  to  prevent 
it  from  exerting  a  poisonous  influence.  The  presence  of  alumi- 
num or  iron  in  an  active  form  is  generally  accompanied  by  a 
high  hydrogen  ion  concentration  due  to  hydrolysis,1  which 
takes  place  readily  in  many  soils. 

186.  Lack  of  nutrients. — Less  is  known  regarding  this 
condition  than  of  the  two  previously  discussed.  The  lack  of 
sufficient  nutritive  calcium  in  an  acid  soil  has  often  been  sug- 
gested.2 In  addition,  it  may  be  possible  that  some  plants  re- 
quire more  calcium  and  other  bases  for  their  metabolic  proc- 
esses when  growing  on  a  so-called  acid  soil,  due  to  the  gen- 
eration of  particular  conditions  within  the  cells.  Plants  like 
alfalfa  absorb  large  amounts  of  calcium  and  may  find  an  acid 
soil  especially  unfavorable  on  this  account. 

True  3  has  shown  that  the  presence  of  calcium  in  consider- 
able amount  is  necessary  when  certain  plants  are  growing  in 
nutrient  solution,  that  other  nutrient  ions  may  penetrate  the 
plant  cells.  Potassium,  for  example,  was  but  slightly  absorbed 
even  when  present  in  large  amounts,  unless  a  certain  concen- 

1  Hydrolysis  is  a  double  decomposition  in  which  one  of  the  inter- 
acting substances  is  water.  The  water  produces  H+  and  OH-  ions, 
the  former  uniting  with  the  non-metallic  portion  of  the  substance  and 
the  hydroxyl  with  the  remainder. 

Active  basic  radicals  give,  with  feeble  acids  in  water,  salts  which 
are  alkaline.  Active  acids  and  active  bases  give  neutral  salts.  Active 
acids  and  less  active  bases  yield  salts  which  are  acid  in  reaction. 

A  feeble  base  and  a  feeble  acid  may  produce  a  salt  which  is  either 
acid  or  alkaline.  Ammonium  sulfide  (NH4)^  in  solution  is  alkaline, 
since  the  ammonium  hydroxide  which  tends  to  form  is  more  dissociated 
than  the  hydrogen  sulfide  which  also  is  present.  Aluminum  silicates  in 
water  hydrolize  readily  and  since  aluminum  hydroxide  is  less  dissociated 
than  silicic  acid,  the  hydrogen  ions  predominate  over  the  hydroxyl  ions 
and  an  acid  reaction  results. 

aSee  Truog,  E.,  Soil  Acidity:  Its  Belation  to  the  Growth  of  Plants; 
Soil  Sci.,  Vol.  V,  No.  3,  pp.  169-195,  1918. 

Also,  Soil  Acidity:  Its  Relation  to  the  Acidity  of  the  Plant  Juices; 
Soil  Sci.,  Vol.  VII,  No.  6,  pp.  469-474,  1919. 

*  True,  R.  H.,  The  Function  of  Calcium  in  the  Nutrition  of  Seed- 
lings; Jour.  Amer.  Soc.  Agron.,  Vol.  13,  No.  3,  pp.  91-107,  1921. 


SOIL  ACIDITY  349 

tration  of  calcium  ions  was  provided.  This  relationship, 
spoken  of  as  synergism,  may  be  seriously  interfered  with  by 
so-called  soil  acidity. 

187.  The  present  status  of  the  question. — Each  of  the 
general  hypotheses  which  have  been  advanced  to  explain  the 
detrimental  influence  of  soil  acidity  has  considerable  plausible 
evidence  in  its  support.  Cane-sugar,  which  is  inverted  only 
in  the  presence  of  an  acid,  was  found  by  Rice  and  Osugi *  to 
be  inverted  in  soils,  even  when  the  water  extracts  from  these 
same  soils  were  neutral  or  even  alkaline.  This  seemed  to  indi- 
cate that  the  acidity  was  actual  and  was  inherent  with  the  soil 
mass  rather  than  with  the  soil  solution.  This  would  also  sug- 
gest the  presence  of  insoluble  or  absorbed  acids  that  might  be 
liberated  by  hydrolysis,  thus  producing  a  harmful  hydrogen 
ion  concentration.  Other  equally  valuable  data  are  available 
on  this  phase  of  soil  acidity.  The  work  of  Hartwell  and  Pem- 
ber  2  and  of  Mirasol,3  however,  is  even  more  conclusive  in  re- 
gard to  aluminum  as  a  toxic  agent,  especially  as  they  studied 
the  problem  from  the  plant  standpoint. 

Conner,4  investigating  the  comparative  influence  of  sulfuric 
acid  and  aluminum  sulfate  on  plants,  has  obtained  some  in- 
teresting data  corroborating  the  work  of  Hartwell  and  Pember. 
By  comparing  a  given  hydrogen  ion  concentration  with  the 
same  hydrogen  ion  concentrations  plus  equivalent  amounts  of 
aluminum  ions,  he  was  able  to  demonstrate  the  greater  toxicity 
of  aluminum  to  barley  and  rye  in  water  culture.  Since  soluble 
aluminum  so  often  accompanies  an  unfavorable  hydrogen  ion 

1  Eice,  F.  E.,  and  Osugi,  S.,  The  Inversion  of  Cane  Sugar  by  Soils 
and  Allied  Substances  and  the  Nature  of  Soil  Acidity;  Soil  Sci., 
Vol.  V,  No.  5,  pp.  333-358,  1918. 

3  Hartwell,  B.  L.,  and  Pember,  F.  E.,  The  Presence  of  Aluminum 
as  a  Reason  for  the  Difference  in  the  Effect  of  So-called  Acid  Soil  on 
Barley  and  Bye;  Soil  Sci.,  Vol.  VI,  No.  4,  pp.  259-277,  1918. 

3  Mirasol,  J.  J.,  Aluminum  as  a  Factor  in  Soil  Acidity;  Soil  Sci., 
Vol.  X,  No.  3,  pp.  153-193,  1920. 

*  Conner,  S.  D.,  Liming  in  Its  Belation  to  Injurious  Inorganic  Com- 
pounds in  the  Soil;  Jour.  Amer.  Soc.  Agron.,  Vol.  13,  No.  3,  pp.  113- 
124,  1921. 


350        NATUKE  AND  PROPERTIES  OF  SOILS 

concentration,  the  importance  of  aluminum  in  acidity  cannot 
be  avoided. 

Table  LXXX 

RELATIVE  WEIGHTS  OF  BARLEY  AND  RYE  GROWN  IN  WATER  CUL- 
TURE.     THE  HYDROGEN  ION  CONCENTRATION  IS  EX- 
PRESSED IN  Ph.1 


Treatment 


Check 

H2S04 j 

A12(S04)3 

H2S04 

A12(S04)3 


H  Ion 

Relative  Weights 

Concentra- 

tion Ph 

Barley 

Rye 

6.3 

100 

100 

4.2 

93 

95 

4.2 

68 

65 

3.9 

73 

65 

3.9 

47 

55 

The  only  conclusion  possible  at  the  present  time  is  that 
there  are  probably  several  kinds  of  acidity  and  many  degrees 
of  the  same  acidity  as  far  as  toxic  influences  are  concerned. 
Moreover,  dissimilar  plants  seem  to  be  affected  differently  by 
the  same  acidity,  while  the  same  plants  respond  diversely  at 
different  times.  Hoagland  2  and  others  3  have  demonstrated 
that  some  plants  grow  better  in  a  slightly  acid  medium,  which 

1  The  hydrogen  ion  concentration  of  an  acid  in  solution  is  a  measure 
of  the  dissociation  of  that  acid  and  of  its  strength.  The  specific  acidity 
of  pure  water  is  taken  as  1,  the  number  of  grams  of  H+  ions  to  a  liter 
being  .0000001  or  10-T.  The  exponent  of  the  power  is  taken  as  an  expres- 
sion of  the  acidity.  Pure  water  has  a  Ph  value  of  7,  which  is  approxi- 
mate neutrality.  An  acid  solution  containing  4000  times  more  H+  ions 
would  have  a  Ph  value  of  3.4. 

2  Hoagland,  D.  R.,  Relation  of  the  Concentration  and  Reaction  of 
the  Nutrient  Medium  to  the  Growth  and  Adsorption  of  the  Plant; 
Jour.  Agr.  Res.,  Vol.  XVIII,  No.  2,  pp.  73-117,  1919. 

3  Gillespie,  L.  J.,  The  Reaction  of  the  Soil  and  Measurements  of 
Hydrogen  ion  Concentration;  Jour.  Wash.  Acad.  Sci.,  Vol.  6,  No.  1, 
pp.  7-16,  1916. 

Sharp,  L.  T.,  and  Hoagland,  D.  R.,  Acidity  and  Adsorption  in  Soils 
as  Measured  by  the  Hydrogen  Electrode;  Jour.  Agr.  Res.,  Vol.  VII, 
No.  3,  pp.  123-145,  1916. 

Gillespie,  L.  J.,  and  Hurst,  L.  A.,  Hydrogen-ion  Concentration — Soil 
Type— Common  Potato  Scab;  Soil  Sci.,  Vol.  VI,  No.  3,  pp.  219-236, 
1918.  '^ 


SOIL  ACIDITY  351 

seems  to  indicate  that  the  hydrogen  ion  concentration  less 
than  a  Ph  value  of  7,  so  often  reported  in  so-called  acid  soils, 
is  concomitant  with  a  toxic  constituent  or  with  malnutrition 
and  is  not  in  itself  the  harmful  agent.1  This  argument,  how- 
ever, does  not  admit  that  the  hydrogen  ion  is  not  in  many 
cases  the  true  explanation  of  the  toxicity  of  certain  acid  soils, 
nor  does  it  suggest  that  lack  of  nutrients  may  not  be  a  serious 
consideration. 

In  light  of  the  explanations  offered  above,  it  is  evident  that 
the  term  soil  acidity  is  inadequate  to  express  the  inorganic 
toxicity  that  accompanies  a  hydrogen  ion  concentration  below 
Ph  7,  as  the  condition  referred  to  is,  in  many  cases,  not  due  to 
the  hydrogen  ion  in  detrimental  concentration.2  Since  the 
term  is  of  long  standing  and  since  so-called  acid  soils  almost 
invariably  yield  an  acid  reaction  with  litmus  paper,  the  phrase 
will  continue  in  use  in  spite  of  its  misleading  inference. 

188.    Why  soil  acidity  develops.3 — No  matter  what  hypoth- 

1  Joffe  found  that  while  alfalfa  plants  experienced  difficulty  in  becom- 
ing established  in  soils  having  high  hydrogen  ion  concentrations  due 
to  the  addition  of  sulfuric  acid,  once  the  seedlings  became  established 
they  showed  normal  color  and  vigor  and  made  excellent  growth  on  soils 
having  a  Ph  value  as  low  as  3.8. 

Joffe,  J.  S.,  The  Influence  of  Soil  Reaction  on  the  Growth  of  Alfalfa; 
Soil  Sci.,  Vol.  X,  No.  4,  pp.  301-307,  1920. 

2  Eesearches  on  Danish  soils  extending  from  1916  to  1920  show  that 
the  Ph  value  on  different  soils  may  vary  from  3.4  to  8.0.  A  rather 
constant  relationship  was  observed  between  the  type  of  vegetation  and 
the  hydrogen  ion  concentration,  many  species  being  found  only  on 
soils  within  a  certain  range  of  Ph  values.  In  water  culture  studies 
so-called  acid-soil  plants  grew  best  at  a  Ph  of  about  4.  Alkaline-soil 
plants  seemed  to  give  the  strongest  growth  at  a  Ph  of  6  to  7. 

Olsen,  C,  The  Concentration  of  the  Hydrogen  Ions  in  the  Soil; 
Science  (N.  S.),  Vol.  LIV,  No.  1405,  pp.  539-541,  Dec.  2,  1921. 

'White,  J.  W.,  Studies  in  Acid  Soils;  Ann.  Eep.  Penn.  State  Col., 
1912-1913,  pp.  55-104. 

Skinner,  J.  J.,  and  Beattie,  J.  H.,  Influence  of  Fertilizers  and  Soil 
Amendments  on  Soil  Acidity;  Jour.  Amer.  Soc.  Agron.,  Vol.  9,  No. 
1,  pp.  25-35,  1917. 

Conner,  S.  D.,  Soil  Acidity  as  Affected  by  Moisture  Conditions  of  the 
Soil;  Jour.  Agr.  Ees.,  Vol.  XV,  No.  6,  pp.  321-329,  1918. 

Martin,  W.  H.,  The  Relation  of  Sulfur  to  Soil  Acidity  and  to  the 
Control  of  Potato  Scab;  Soil  Sci.,  Vol.  IX,  No.  6,  pp.  393-408,  1920. 


352         NATURE  AND  PROPERTIES  OF  SOILS 

esis  may  be  considered  as  best  explaining  soil  acidity,  sci- 
entific and  practical  men  are  agreed  that  the  addition  of  cer- 
tain compounds  of  calcium  and  magnesium  tend  to  alleviate 
the  detrimental  condition.  Conversely,  almost  every  one  is 
willing  to  admit  that  the  most  reasonable  causejjjLiU  develop- 
ment is  the  loss  or  inactivity  of  certain  liases.  A  lack  of  cal- 
cium seems  especially  prone  to  allow  an  increased  hydrogen 
ion  concentration  to  develop  and  may  at  the  same  time  en- 
courage the  activity  of  certain  toxic  bases  or  produce  malnu- 
trition. The  tendency  of  all  soils  in  a  humid  region  is,  there- 
fore, towards  acidity,  their  condition  depending  on  the  activ- 
ity of  certain  factors  which  seem  to  produce  such  a  condition. 
The  four  important  factors  generally  specified  as  encour- 
aging acidity  are:  (1)  leaching  losses,  (2)  cropping  losses, 
(3)  absorption  phenomena  within  the  soil,  and  (4)  fertilizer 
residues. 

The  loss  of  nutrient  bases  from  the  soil  has  already  been 
emphasized  (par.  163)  and  the  importance  of  such  removal  is 
evident  from  the  standpoint  of  plant  nutrition.  Over  a  period 
of  ten  years,  the  removal  of  nutrients  from  the  Cornell  lysi- 
meter  soils,1  by  drainage  and  rotation  cropping  together, 
amounted  to  3702,  1741,  and  942  pounds  to  the  acre,  respec- 
tively, for  lime  (CaO),  potash  (K20),  and  magnesia  (MgO). 
The  loss  of  such  amounts  of  bases  cannot  but  permit  the  rapid 
development  of  soil  acidity.  No  matter  how  well  supplied 
the  soil  may  be  with  favorable  bases,  it  will  in  time  become 
acid. 

Absorption,  in  its  influence  on  soil  acidity,  produces  its 
effect  by  rendering  certain  bases  inactive  rather  than  by 
removing  them  from  the  soil.  When  the  activity  of  such  bases 
as  calcium  is  reduced  by  absorptive  influences,  not  only  does 
the  hydrogen  ion  concentration  of  the  soil  solution  tend  to  in- 
crease, but  the  hydrolysis  of  compounds  carrying  aluminum 
and  similar  bases  seems  to  be  encouraged.  The  acidity  as  de- 
1  Unpublished  data,  Cornell  Agr.  Exp.  Sta.,  Ithaca,  N.  Y. 


SOIL  ACIDITY  353 

veloped  may  have  a  nutritive  relationship  as  well  as  a  toxic 
effect. 

When  fertilizer  salts  are  added  to  the  soil,  the  basic  ions 
are  usually  absorbed  to  a  greater  degree  than  the  acid  radi- 
cals. This  tends  to  develop  actual  acidity  in  the  soil  solution, 
which  may  in  itself  be  toxic  or  may  facilitate  the  development 
of  detrimental  ions.  If  the  crop  utilizes  the  basic  ion  of  the 
fertilizer  added  to  a  greater  extent  than  the  acid  radical,  it 
will  aid  in  the  development  of  acidity.  If  the  plant,  on  the 
other  hand,  absorbs  the  acid  radical,  it  will  tend  to  counter- 
act the  selective  absorption  by  the  soil.  The  combined  influ- 
ences of  soil  and  crop  on  ammonium  sulfate  tend  to  develop 
acidity,  while  the  effect  on  sodium  nitrate  is  toward  alkalinity. 
A  salt  such  as  potassium  nitrate  should  leave  no  residue. 

The  decomposition  of  organic  matter,  especially  when  green- 
manures  are  plowed  under,  is  often  considered  as  increasing 
the  acidity  of  the  soil.  Such  may  be  the  case  at  the  beginning 
of  the  decomposition  process,  but  the  data1  available  on  the 
subject  seem  to  indicate  that  organic  matter,  if  it  exerts  any 
influence  on  acidity,  tends  to  reduce  rather  than  accentuate 
it.  This  result  may  occur  through  the  liberation  of  bases 
from  the  organic  matter  as  decomposition  proceeds. 

189.  Relative  tolerance  of  acidity  by  plants. — Since  so 
many  intermediate  influences  are  possible  in  acid  soils,  and 
since  plants  respond  so  differently  to  these  influences,  it  is  im- 
possible to  forecast  the  relative  resistance  of  different  crops 
on  the  same  soil.  The  response  of  the  same  crop  on  differeux 
acid  soils  is  likewise  difficult  to  foretell. 

It  is  known  that  certain  crops  are  often  more  tolerant  to 
soil  acidity  than  others.    Of  the  common  weeds  sheep  sorrel, 

1  White,  J.  W.,  Soil  Acidity  as  Influenced  by  Green  Manures;  Jour. 
Agr.  Res.,  Vol.  XIII,  No.  3,  pp.  171-197,  1918. 

Stephenson,  E.  E.,  The  Effect  of  Organic  Matter  on  Soil  Beaction; 
Soil  Sci.,  Vol.  VI,  No.  6,  pp.  413-439,  1918. 

Howard,  L.  P.,  The  Beaction  of  the  Soil  as  Influenced  by  the  De- 
composition of  Green  Manures;  Soil  Sci.,  Vol.  IX,  No.  1,  pp.  27-38, 
1920. 


354        NATURE  AND  PROPERTIES  OF  SOILS 

paint-brush,  daisy,  and  plantain  seem  especially  resistant. 
This  does  not  mean,  however,  that  they  grow  better  on  an 
extremely  acid  soil  than  on  one  that  is  slightly  acid  or  neutral. 
Some  of  the  common  crops  that  are  tolerant  of  acidity  are 
strawberry,  blackberry,  watermelon,  red-top,  Rhode  Island 
bent-grass,  cowpea,  soybean,  rye,  millet,  and  buckwheat.  Such 
crops  as  alfalfa,  red  clover,  timothy,  maize,  oats,  barley,  cab- 
bage and  sugar-beet  seem  to  be  susceptible  in  various  degree 
to  acid  conditions. 

Reasons  for  the  above  differences  are  not  as  yet  known,  since 
plants  apparently  alike  in  every  other  respect  differ  in  their 
reaction  to  the  same  acid  condition.  The  following  pairs  of 
plants  may  be  listed  as  examples:  watermelon  and  musk- 
melon,  blackberries  and  raspberries,  apple  and  quince,  turnip 
and  beet,  beans  and  alfalfa,  red-top  and  timothy,  rye  and 
barley.  The  first  of  each  pair  mentioned  will  grow  well  on 
acid  soils,  while  the  second  crop  in  each  case  is  very  detri- 
mentally affected.1 

190.  Tests  for  soil  acidity.2 — The  great  importance  of 
soil  acidity  to  plant  growth  has  directed  much  attention 
towards   methods   for   determining   the   acidity   of  the   soil. 

1  Hartwell,  B.  L.,  Need  for  Lime  as  Indicated  by  Belative  Toxicity  of 
Acid  Soil  Conditions  to  Different  Crops;  Jour.  Amer.  Soc.  Agron.,  Vol. 
13,  No.  3,  pp.  108-112,  1921. 

2  Some  of  the  important  methods  are  compared  and  discussed  in  the 
following  articles: 

Schollenberger,  C.  J.,  Belation  Between  the  Indications  of  Several 
Lime-requirement  Methods  and  the  Soil  Content  of  Bases;  Soil  Sci., 
Vol.  Ill,  No.  3,  pp.  279-288,  1917. 

Christensen,  H.  E.,  Experiments  in  Methods  for  Determining  the 
Beaction  of  Soils;  Soil  Sci.,  Vol.  IV,  No.  2,  pp.  115-178,  1917. 

Stephenson,  E.  E.,  Soil  Acidity  Methods;  Soil  Sci.,  Vol.  VI,  No.  1, 
pp.  33-52,  1918. 

Blair,  A.  W.,  and  Prince,  A.  L.,  The  Lime  Bequirement  of  Soils 
According  to  the  Veitch  Method,  Compared  with  the  Hydrogen-Ion 
Concentration  of  the  Soil  Extract;  Soil  Sci.,  Vol.  IX,  No.  4,  pp.  253- 
259,  1920. 

Hartwell,  B.  L.,  Pember,  F.  Ev  and  Howard,  L.  P.,  Lime  Bequire- 
ment as  Determined  by  the  Plant  and  by  the  Chemist;  Soil  Sci.,  Vol. 
VII,  Nc.  4,  pp.  279-282,  1919. 


SOIL  ACIDITY  355 

Such  methods  may  be  divided,  for  convenience  of  discussion, 
under  two  heads:  quantitative  determinations  and  qualita- 
tive tests.  In  the  first  case  the  methods  devised  purport  to 
give  the  lime  requirement  of  the  soil.  The  second  group  of 
methods  attempts  to  determine  whether  the  soil  is  acid  and 
may  in  addition  give  some  general  idea  as  to  the  degree  of 
acidity. 

191.  Lime-requirement  determinations. — A  great  num- 
ber of  methods  has  been  advanced  for  the  determination  of  the 
lime  requirement  of  soils.  The  methods  may  for  convenience 
be  grouped  under  three  heads:  (1)  those  using  a  neutral  salt,1 
(2)  those  utilizing  a  basic  substance,2  and  (3)  miscellaneous 
procedures. 

In  the  first  group,  some  neutral  salt  such  as  potassium  ni- 
trate is  added  to  the  soil  and  the  amount  of  actual  acidity 
developed  is  determined  under  suitable  control.  The  actual 
acidity  produced  by  selective  absorption  and  basic  exchange 
is  thus  taken  as  a  measurement  of  the  soil  acidity  and  is  gen- 
erally figured  to  pounds  of  lime  to  the  acre. 

In  the  second  group  some  basic  substance,  preferably  that 
which  is  used  in  practice  to  correct  acidity,  is  added  to  the 
soil.  The  amount  of  the  basic  substance  necessary  to  render 
the  soil  alkaline  or  neutral  is  determined  in  pounds  to  the 


1  The  Hopkins  methods  utilize  potassium  nitrate  or  sodium  chloride. 
Calcium  acetate  is  used  in  the  Jones  method. 

Hopkins,  C.  G.,  Knox,  W.  H.,  and  Pettit,  J.  H.,  A  Quantitative 
Method  for  Determining  the  Acidity  of  Soils;  U.  S.  Dept.  Agr.,  Bur. 
Chem.,  Bui.  73,  pp.  114-121,  1903. 

Jones,  C.  H.,  Method  for  Determining  the  Lime  Requirement  of 
Soils;  Jour.  Assoc.  Off.  Agr.  Chemists,  Vol.  I,  No.  1,  pp.  43-44,  1915. 

2  The  Veitch  method  utilizes  calcium  hydroxide,  the  Tacke  method 
calcium  carbonate  and  the  method  proposed  by  Hutchinson  and  Mac- 
Lennan  calcium  bicarbonate. 

Veitch,  P.  P.,  Comparison  of  the  Methods  for  the  Estimation  of 
Soil  Acidity;  Jour.  Amer.  Chem.  Soc,  Vol.  26,  pp.  637-662,  1904. 

Tacke,  Br.,  iioer  die  Bestimmung  der  freien  Humussduren;  Chem. 
Ztg.,  Bd.  21,  Heft.  20,  S.  174-175,  1897. 

Hutchinson,  H.  B.,  and  MacLennan,  K.,  The  Determination  of  the 
Lime  Requirement  of  the  Soil;  Chem.  News,  Vol.  110,  p.  61,  1914. 


356        NATURE  AND  PROPERTIES  OF  SOILS 

acre.  Calcium  hydroxide  and  calcium  carbonate  are  often 
used. 

Many  investigators  consider  that  the  hydrogen  ion  concen- 
tration of  the  soil  solution  is  a  fair  measure  of  the  lime  re- 
quirement of  a  soil.1  They  thus  assume  that  the  concentra- 
tion of  the  hydrogen  ion  is  a  comparative  indication  of  the 
amount  of  lime  necessary  to  alleviate  the  detrimental  influ- 
ences due  to  acidity.  Bouyoucos 2  claims  that  the  depression 
of  the  freezing  point  (see  par.  145)  may  be  used  to  measure 
soil  acidity.  He  found  that  the  depression  of  the  freezing 
point  was  less  for  a  neutral  soil  than  for  one  either  acid  or 
alkaline. 

192.  The  Veitch  method. — In  order  to  show  something 
of  the  procedure  necessary  in  determining  the  lime  require- 
ment of  the  soil,  the  Veitch  method,  which  utilizes  calcium 
hydroxide,  will  be  briefly  described.  Eleven  and  and  one-fifth 
grams  of  soil  are  placed  in  a  suitable  Erlenmeyer  flask  and 
treated  with  a  standard  lime-water  solution.  The  amount  of 
soil  taken  and  the  strength  of  the  calcium  hydroxide  solution 
are  such  that  each  cubic  centimeter  of  the  latter  absorbed  by 
the  soil  indicates  the  need  of  300  pounds  of  calcium  oxide 
to  the  acre.  A  number  of  samples  are  run  at  the  same  time, 
receiving  progressively  larger  amounts  of  lime-water.     The 

1  Gainey,  P.  L.,  Soil  Beaction  and  Growth  of  Azotobacter;  Jour. 
Agr.  Res.,  Vol.  XIV,  No.  7,  pp.  265-271,  1918. 

Gillespie,  L.  J.,  and  Hurst,  L.  A.,  Hydrogen  Ion  Concentration — 
Soil  Type — Common  Potato  Scab;  Soil  Sci.,  Vol.  VI,  No.  3,  pp.  219- 
236,  1918. 

Plummer,  J.  K.,  Studies  in  Soil  Reaction  as  Indicated  by  the  Hydro- 
gen Electrode;  Jour.  Agr.  Res.,  Vol.  XII,  No.  1,  pp.  19-31,  1918. 

Joffe,  J.  H.,  Hydrogen  Ion  Concentration  Measurements  in  Soils  in 
Connection  with  Their  Lime  Requirements ;  Soil  Sci.,  Vol.  IX,  No.  4, 
pp.  261-266,  1920. 

Blair,  A.  W.,  and  Prince,  A.  L.,  The  Lime  Requirement  of  Soils 
According  to  the  Veitch  Method  Compared  with  the  Hydrogen  Ion  Con- 
centration of  the  Soil  Extract;  Soil  Sc:.,  Vol.  IX,  No.  4,  pp.  253-259, 
1920. 

2  Bouyoucos,  G.  J.,  The  Freezing  Point  Method  as  a  New  Means  of 
Determining  the  Nature  of  Acidity  and  Lime  Requirements  of  Soils; 
Mich.  Agr.  Exp.  Sta.,  Tech.  Bui.  27,  1916. 


SOIL  ACIDITY  357 

samples  are  brought  to  dryness  over  a  steam  bath  and  then 
taken  up  with  about  100  cubic  centimeters  of  water.  The 
samples,  after  shaking,  are  allowed  to  settle,  and  the  super- 
natant liquid  is  treated  with  phenolphthalein.  By  the  use 
of  a  number  of  samples  with  varying  amounts  of  lime-water, 
the  amount  of  the  reagent  necessary  to  neutralize  the  soil  can 
be  approximately  determined. 

The  objections  that  can  be  urged  against  the  Veitch  method 
may  serve  to  indicate  the  difficulties  that  are  in  general  en- 
countered in  using  most  of  the  methods  for  determining  the 
lime  requirement  of  soils.  The  method  is,  in  the  first  place, 
very  artificial,  there  being  no  assurance  that  the  amount  of 
calcium  absorbed  is  the  same  as  that  necessary  to  neutralize 
the  soil  under  field  conditions.  In  the  second  place,  it  is 
subject  to  considerable  error.  Even  with  the  most  careful 
manipulation,  the  method  is  hardly  accurate  within  300 
pounds  of  calcium  oxide  to  the  acre. 

If  the  results  from  such  a  method  are  to  be  applied  directly 
to  practical  liming  it  must  be  assumed  that  the  amount  of  lime 
necessary  to  neutralize  an  acid  soil  is  the  same  as  that  capable 
of  alleviating  the  acidity  for  a  particular  crop.  In  light 
of  the  variable  influences  of  acidity  on  plants,  this  is  an  un- 
scientific assumption  to  say  the  least.  Acidity  itself  is  too 
intangible  a  condition.  Moreover,  it  is  in  many  cases  not  only 
inadvisable  but  also  unprofitable  to  satisfy  the  full  lime  re- 
quirement of  a  soil.  Some  crops  are  unharmed  or  may  even 
be  benefited  by  moderate  acidity.  The  selection  of  a  lawn 
grass,  for  example,  which  is  tolerant  to  acidity  may  allow 
the  suppression  of  certain  troublesome  weeds  that  would 
spring  up  if  the  soil  was  limed. 

Since  the  results  from  lime-requirement  methods  must  be 
so  radically  modified  to  suit  field  conditions,  they  seem  but 
little  better  in  a  practical  way  than  qualitative  tests,  which 
distinguish  only  in  a  general  manner  between  different  de- 
grees of  acidity.    The  rapidity  and  simplicity  of  qualitative 


358        NATURE  AND  PROPERTIES  OF  SOILS 

tests  give  them  an  advantage  over  the  somewhat  questionable 
lime-requirement  determinations.  As  the  amount  of  lime 
applied  is  at  best  only  an  estimate,  a  simple  test,  rationally 
correlated  with  the  many  other  factors  that  must  be  consid- 
ered, may  prove  as  satisfactory  as  a  more  complicated  pro- 
cedure. 

193.  Qualitative  tests  for  acidity — litmus  paper. — Per- 
haps the  oldest  test  for  acidity  is  the  use  of  litmus  paper.1 
This  may  be  used  alone  or  in  connection  with  some  sensitiz- 
ing agent.  Potassium  nitrate,  a  neutral  salt,  is  often  utilized 
in  this  capacity.  As  has  already  been  explained  (par.  141), 
the  addition  of  such  a  salt,  especially  to  a  soil  lacking  in  ac- 
tive bases,  results  in  a  marked  selective  absorption  and  the 
development  of  a  hydrogen  ion  concentration.  In  using  litmus 
paper  and  potassium  nitrate  it  is  assumed  that  the  selective 
absorption  and  basic  exchange  is  an  approximate  measure  of 
the  so-called  soil  acidity. 

The  procedure  is  as  follows:  A  small  amount  of  the  soil 
to  be  tested  is  placed  in  a  small  dish  or  other  container  and 
moistened  with  a  neutral  potassium  nitrate  solution.  A  thick 
batter  is  produced  by  mixing.  The  soil  is  then  smoothed 
down  and  one  end  of  a  strip  of  neutral  litmus  paper  is  care- 
fully applied.  The  reddening  of  the  paper  is  an  indication 
of  acidity,  while  the  rate  of  the  reaction  is  a  rough  measure 
of  the  degree.  The  portion  of  the  paper  not  in  contact  with 
the  soil  may  be  used  for  comparison  when  the  change  is  slight. 
The  unused  end  may  even  be  moistened  with  distilled  water 
to  make  the  comparison  more  accurate. 

194.  The  zinc-sulfide  test. — Another  qualitative  test 
based  on  the  same  general  principles  has  more  recently  been 

1  Barlow,  J.  T.,  Soil  Acidity  and  the  Litmus  Paper  Method  for  Its 
Detection;  Jour.  Amer.  Soc.  Agron.,  Vol.  8,  No.  1-  pp.  23-30,  1916. 

Karraker,  P.  E.,  The  Value  of  Blue  Litmus  Paper  from  Different 
Sources  as  a  Test  for  Soil  Acidity;  Jour.  Amer.  Soc.  Agron.,  Vol.  10, 
No.  4,  pp.  180-182,  1918. 


SOIL  ACIDITY  359 

developed.  This  is  the  zinc-sulfide  method.1  The  soil  sample, 
usually  10  grams,  is  placed  in  an  Erlenmeyer  flask  and  treated 
with  an  excess  of  neutral  calcium  chloride  and  zinc  sulfide. 
About  75  cubic  centimeters  of  water  are  added.  The  mixture 
is  boiled  for  one  minute  to  control  frothing  and  to  develop 
uniform  ebullition.  A  strip  of  moistened  lead  acetate  paper 
is  now  laid  over  the  mouth  of  the  flask  and  allowed  to  remain 
there  exactly  three  minutes,  the  boiling  being  continued  at 
a  uniform  rate.  The  reactions  involved  in  the  test  are  as  fol- 
lows : 

Soil  +  xCaCl2  (neutral)  ±5  Cax  Soil  +  xHCl 

2HC1  +  ZnS  =  ZnCl2  +  H2S 

H2S  (Expelled  by  boiling)  +  Pb(C2H302)2  =  PbS 

(black)  +  2C2H402 

The  selective  absorption  and  basic  exchange  of  the  soil  de- 
velops actual  acidity,  which  produces  hydrogen  sulfide  from 
the  zinc  sulfide.  The  gas  is  driven  off  against  the  lead  acetate 
paper,  producing  a  black  color.  The  principle  involved  is  the 
same  as  that  already  explained  for  the  litmus  test,  a  different 
means  being  employed  for  measuring  the  actual  acidity  de- 
veloped. 

195.  Comparison  and  criticism  of  qualitative  tests. — A 
comparison  and  criticism  of  these  two  methods  will  amply 
show  the  advantages  and  disadvantages  of  qualitative  tests  2 

1  Truog,  E.,  New  Method  for  the  Determination  of  Soil  Acidity; 
Science,  N.  S.,  Vol.  40,  pp.  246-248,  1914. 

Truog,  E.,  Testing  Soils  for  Acidity;  Wis.  Agr.  Exp.  Sta.,  Bui.  312, 
1920. 

2  There  are  a  number  of  other  qualitative  tests  for  acidity,  of  which 
the  following  may  be  mentioned: 

Ammonia  test. — In  this  test  the  soil  is  placed  in  a  bottle  and  treated 
with  a  strong  solution  of  ammonia.  After  shaking,  the  soil  is  allowed 
to  settle,  the  depth  of  the  color  developing  in  the  supernatant  liquid 
being  considered  as  indicating  the  degree  of  acidity.  This  color  depends 
on  the  amount  and  character  of  the  soil  organic  matter  rather  than  on  the 
acidity. 

Acid  test  for  carbonates. — In  this  test  a  sample  of  the  soil  is  treated 
with  a  few  drops  of  dilute  hydrochloric  acid.    Effervescence  indicates  the 


360        NATURE  AND  PROPERTIES  OF  SOILS 

in  general.  The  litmus  paper  test  is  simple  and  rapid.  It 
can  be  used  with  equal  facility  in  the  laboratory  and  field. 
While  its  readings  may  not  correlate  very  definitely  with  the 
actual  amount  of  lime  that  should  be  applied,  it  gives  a  basis 
for  an  estimate  that  in  practice  should  include  a  number  of 
factors  besides  so-called  soil  acidity.  One  objection  to  the 
method  lies  in  the  difficulty  of  obtaining  sensitive  litmus  paper. 
Again  the  intensity  of  the  color  change  is  not  great  and  in  the 
hands  of  an  inexperienced  person  may  seem  insignificant.  In 
spite  of  its  limitations,  it  is  one  of  the  best  practical  qualita- 
tive tests  for  soil  acidity  now  available. 

The  zinc-sulfide  test  is  much  more  striking  than  the  litmus 
test  and  thus  is  more  easily  interpreted.  On  account  of  the 
marked  change  of  color  there  is  always  a  temptation  to  read 
into  this  test  a  quantitative  value  which  it  does  not  possess 
to  any  greater  degree  than  does  the  litmus  paper  method. 

The  zinc  sulfide  test  is  not  as  rapid  as  the  litmus  test,  nor  is 
it  a  satisfactory  field  method.  Moreover,  it  is  more  complex 
and  requires  a  much  more  extensive  technique.  Again  it  does 
not  distinguish  between  a  neutral  and  an  alkaline  soil.  Lit- 
mus paper,  on  the  other  hand,  indicates  alkalinity  and  acidity 
with  equal  facility.  The  zinc-sulfide  test  is  not  a  method 
suited  for  those  inexperienced  in  laboratory  procedure.  The 
deductions  from  the  two  tests,  however,  should  be  approxi- 
mately the  same. 

196.  Resume. — Soil  acidity  is  a  more  or  less  unfavorable 
biological  condition,  which  develops  in  soils  due  to  the  lack  or 

presence  of  sufficient  favorable  bases  in  the  carbonate  or  bicarbonate 
forms.    A  soil,  however,  may  be  alkaline  and  yet  fail  to  effervesce. 

Potassium  sulfo-cyanate  test. — A  new  test  has  recently  been  proposed 
in  which  a  sample  of  soil  held  in  a  test-tube  is  treated  with  an  alcoholic 
solution  of  potassium  sulfo-cyanate  (KSCN).  If  the  supernatant  liquid 
turns  red,  soluble  iron  is  present,  the  degree  of  color  indicating  the 
amount.  It  is  assumed  that  the  soluble  iron  is  a  comparative  measure 
of  the  active  aluminum  in  the  soil  and  that  aluminum  is  the  toxic 
constituent. 

Comber,  N.  M.,  A  Qualitative  Test  for  Sour  Soils;  Jour.  Agr.  Sci., 
Vol.  10,  part  4,  pp.  420-424,  1920. 


SOIL  ACIDITY  361 

inactivity  of  certain  bases,  especially  those  which  tend  to- 
wards soil  alkalinity.  These  necessary  bases  may  be  rendered 
inactive  by  absorption  phenomena  or  may  be  actually  lost 
through  leaching  and  cropping.  The  specific  and  usually  de- 
leterious influence  of  so-called  soil  acidity  may  be  due  to  an 
excessive  hydrogen  ion  concentration  or  to  toxic  bases  such 
as  aluminum,  iron,  and  manganese,  which  become  active  when 
ionic  calcium  and  similar  bases  are  lacking,  thus  encouraging 
a  hydrogen  ion  accumulation.  It  is  not  improbable  that  in 
some  cases  the  detrimental  influence  may  be  improper  nutri- 
tion, either  due  to  a  lack  of  calcium  as  a  nutrient  or  as  a  syner- 
gistic agent  necessary  for  the  absorption  of  other  nutrients 
by  plants.  These  detrimental  conditions  are  alleviated  in 
practice  by  the  application  of  some  form  of  lime. 

A  number  of  different  methods  has  been  devised  to  ascer- 
tain quantitatively  the  lime  requirements  of  soils.  They  are 
all  more  or  less  inaccurate.  Moreover,  the  lime  requirement 
of  a  soil  and  the  lime  necessary  for  best  plant  growth  on  that 
soil  are  not  of  necessity  the  same.  Plants  respond  very  differ- 
ently to  the  diverse  conditions  that  may  develop  in  the  various 
acid  soils  and  it  is  seldom  necessary  or  practicable  entirely  to 
neutralize  a  very  acid  soil  in  order  to  correct  its  deleterious 
condition.  While  lime-requirement  methods  are  valuable  in 
research,  qualitative  tests  are  sufficient  in  practice.  The 
amount  of  lime  that  should  be  applied  is  determined  not  only 
by  the  degree  and  nature  of  the  acidity  but  also  by  the  char- 
acter of  the  crops,  the  length  of  rotation,  the  system  of  fer- 
tilization, and  similar  factors.  At  best  the  amount  of  lime 
that  should  be  applied  to  the  acre  is  but  an  estimate  based 
on  many  conditions,  of  which  acidity  is  one.  A  qualitative 
test  seems  as  satisfactory  a  basis  for  such  an  estimate  as  a 
more  carefully  controlled  quantitative  determination. 


CHAPTER  XIX 
LIMING  THE  SOIL » 

While  soil  acidity  is  a  condition  but  imperfectly  under- 
stood, most  investigators  are  agreed  that  it  is  due  to  a  lack  or 
inactivity  of  certain  bases,  especially  those  that  tend  to  reduce 
the  hydrogen  ion  concentration  of  the  soil  solution  and  to 
give  the  soil  an  alkaline  reaction.  The  correction  of  acidity 
obviously  lies  in  the  addition  of  compounds  which  carry  the 
necessary  bases  in  such  forms  that  the  acidity  may  be  partially 
or  wholly  alleviated. 

The  base  most  commonly  used  to  correct  acidity  is  calcium, 
although  magnesium  is  often  applied,  especially  in  connec- 

1  The  following  publications  may  be  of  interest : 

Hopkins,  C.  G.,  Ground  Limestone  for  Acid  Soils;  111.  Agr.  Exp.  Sta., 
Circ.  110,  1907. 

Ellett,  W.  B.,  Lime  for  Virginia  Farms;  Va.  Agr.  Exp.  Sta.,  Bui.  187, 
1910. 

Brown,  P.  E.,  Bacteriological  Studies  of  Field  Soils:  The  Effects  of 
Lime;  la.  Agr.  Exp.  Sta.,  Res.  Bui.  5,  1912. 

Whitson,  A.  R.,  and  Weir,  W.  W.,  Soil  Acidity  and  Liming;  Wis.  Agr. 
Exp.  Sta.,  Bui.  230,  1913. 

Frear,  W.,  Sour  Soils  and  Liming;  Penn.  Dept.  Agr.,  Bui.  261, 
1915. 

Miller,  M.  F.,  and  Krusekopf,  H.  H.,  Agricultural  Lime;  Mo.  4gr- 
Exp.  Sta.,  Bui.  146,  1917. 

Mooers,  C.  A.,  Ground  Limestone  and  Prosperity;  Tenn.  Agr.  Exp. 
Sta.,  Bui.  119,  1917. 

Shorey,  E.  C,  The  Principles  of  the  Liming  of  Soils;  U.  S.  Dept.  Agr., 
Farmers'  Bui.  921,  1918. 

McCool,  M.  M.,  and  Millar,  C.  E.,  Some  General  Information  on  Lime 
and  Its  Uses  and  Functions  in  Soils;  Mich.  Agr.  Exp.  Sta.,  Special  Bui. 
91,  1918. 

Agee,  Alva.,  The  Bight  Use  of  Lime  in  Soil  Improvement;  New  York, 
1919. 

Hudelson,  R.  R.,  Keeping  Soils  Productive;  Mo.  Agr.  Exp.  Sta.,  Circ. 
102,  1921. 

362 


LIMING  THE  SOIL  363 

tion  with  calcium.  Calcium  is  employed  because  it  is  not  only- 
effective  with  all  types  of  acidity  but  because  it  is  compara- 
tively cheap  and  plentiful.  Potassium  in  active  form  is  too 
expensive,  sodium  is  likely  to  generate  harmful  compounds 
in  the  soil,  while  magnesium  in  large  amounts  is  sometimes 
harmful.  Calcium  compounds  may  be  applied  in  excess  and 
yet  no  harmful  effects  on  plant  growth  are  ordinarily  likely 
to  result.1 

197.  Forms  of  lime. — The  term  lime  correctly  used  re- 
fers only  to  calcium  oxide  (CaO).  In  a  popular  and  agri- 
cultural sense  the  scope  of  the  word  has  been  broadened  to 
include  all  of  the  commercial  compounds  of  calcium  and  mag- 
nesium commonly  applied  to  the  soil  to  correct  the  so-called 
acidity.  The  term  in  its  agricultural  sense  refers  to  the  fol- 
lowing compounds  either  alone  or  in  mixture:  calcium  oxide 
(CaO),  magnesium  oxide  (MgO),  calcium  hydroxide  (Ca- 
(OH)2),  magnesium  hydroxide  (Mg(OH)2),  calcium  car- 
bonate (CaC03),  and  magnesium  carbonate  (MgC03).  Such 
compounds  as  gypsum  (CaS04.2H20),  mono-calcium  phos- 
phate (CaH4(P04)2),  and  calcium  silicate  (Ca2Si04),  insofar 
as  they  are  carriers  of  calcium,  also  might  be  spoken  of  as  lime. 

As  might  be  expected,  liming  materials  do  not  appear  on  the 
market  as  single  compounds  of  magnesium  or  calcium,  nor 
are  they  by  any  means  pure.  The  better  grades  of  the  oxides 
and  hydroxides  are  generally  used  in  the  trades,  the  more  im- 
pure materials  having  an  outlet  as  agricultural  lime.  The  car- 
bonated forms  of  lime  have  a  number  of  different  sources  and 
vary  to  a  marked  degree  in  purity.  Lime,  in  whatever  form 
it  may  appear  on  the  market,  almost  always  carries  magnesium 
as  well  as  calcium,  the  latter  usually  predominating. 

Three  general  groups  of  lime,  as  it  is  commercially  handled, 


1  Floyd,  B.  F.,  Some  Cases  of  Injury  to  Citrus  Trees  Apparently 
Induced  by  Ground  Limestone;  Fla.  Agr.  Exp.  Sta.,  Bui.  137,  1917. 

Wyatt,  F.  A.,  Influence  of  Calcium  and  Magnesium  Compounds  on 
Plant  Growth;  Jour.  Agr.  Bes.,  Vol.  VI,  No.  16,  pp.  589-619,  1916. 


364        NATUKE  AND  PROPERTIES  OF  SOILS 

may  be  recognized:  (1)  burned  lime,1  (2)  water-slaked  or 
simply  slaked  lime,2  and  (3)  carbonated  lime.3 

The  devices  for  producing  burned  lime  are  various,  rang- 
ing from  the  farmer's  lime  heap  to  the  immense  cylindrical 
kilns  of  commerce.  In  any  case  the  general  result  is  the  same. 
The  limestone  with  which  the  kiln  is  charged  is  decomposed  by 
the  heat,  carbon  dioxide  and  other  gases  are  discharged,  and 
calcium  and  magnesium  oxides  are  left  behind.4  The  purity  of 
burned  lime,  as  it  is  sold  for  agricultural  purposes,  is  quite 
variable,  ranging  from  60  to  98  per  cent,  of  calcium  and  mag- 
nesium oxides.  As  high  as  40  per  cent,  of  burned  lime  may 
be  magnesium  oxide,  if  the  original  stone  was  dolomitic.  The 
impurities  of  burned  lime  consist  of  the  original  impurities 
of  the  limestone,  such  as  chert,  clay,  iron  compounds,  and  the 
like,  as  well  as  unburned  fragments  of  the  stone.  These  ma- 
terials are  often  partially  screened  out  before  the  product  ap- 
pears on  the  market. 

Slaked  lime  is  produced  by  adding  water  to  the  burned 
product,  a  hydroxide  resulting  from  the  direct  union  of  the 
oxides  of  calcium  and  magnesium  with  water.5  Often  some 
of  the  calcium  and  magnesium  oxides  remain  unslaked.  Four 
lime  compounds  may,  therefore,  appear  in  freshly  slaked  lime, 
besides  the  original  impurities  of  the  burned  materials.    Com- 

1  Often  spoken  of  as  burnt  lime,  oxide  of  lime  and  quick  lime.  It 
may  be  purchased  either  in  the  lump  form  or  in  a  finely  ground  condi- 
tion.   It  is  highly  caustic  and  reacts  readily  with  water. 

"Incorrectly  designated  in  trade  as  hydrated  lime  or  lime  hydrate. 
It  is  strongly  alkaline  and  quite  caustic  but  not  to  the  degree  exhibited 
by  calcium  and  magnesium  oxides.  Calcium  hydroxide  and  magnesium 
hydroxide  are  soluble  in  cold  water  to  the  extent  of  about  17  parts  and 
.09  parts  in  10,000,  respectively. 

3  The  carbonated  forms  of  lime  are  often  incorrectly  spoken  of  as 
lime  carbonate  and  carbonate  of  lime.  Calcium  and  magnesium  carbo- 
nates are  soluble  in  pure  cold  water  to  the  extent  of  only  about  .13  and 
1.06  parts  in  10,000,  respectively.  The  reaction  to  litmus  is  slightly 
alkaline. 

4CaCO,  +  Heat  =  CaO  +  C02. 
MgCO,  +  Heat  =  MgO  +  CO,. 
•CaO  +  H20=Ca(OH)a. 
MgO  +  HaO  =  Mg(OH),. 


LIMING  THE  SOIL  365 

mercial  slaked  lime  ranges  in  composition  from  60  to  75  per 
cent,  of  lime  expressed  as  calcium  plus  magnesium  oxides. 
Both  the  burned  and  slaked  forms  of  lime  tend  to  absorb  car- 
bon dioxide  from  the  air,  producing  calcium  and  magnesium 
carbonate.    This  is  called  air-slaking.1 

A  number  of  lime  compounds  are  sold  under  the  head  of 
carbonated  lime.  Of  these  pulverized  or  ground  limestone  is 
the  most  common.  There  is  also  bog  lime  or  marl,  oyster 
shells  and  artificial  carbonates.  The  latter  are  by-products 
from  certain  industries.  All  of  these  are  quite  variable  in 
their  content  of  calcium  and  magnesium  carbonates.  Pul- 
verized limestone  may  vary  in  purity  from  75  to  98  per  cent., 
90  per  cent,  being  a  fair  average.  Highly  magnesian  stone  is 
generally  avoided,  although  stone  carrying  from  15  to  20  per 
cent,  of  magnesium  carbonate  is  often  used.  The  magnesium 
carbonate,  however,  usually  makes  up  less  than  5  per  cent,  of 
the  lime  present. 

The  figures 2  quoted  in  table  LXXI  (see  page  366)  show  the 
average  composition  of  liming  materials  offered  for  sale  in 
Pennsylvania  from  1916  to  1920  inclusive. 

198.  Determining  the  need  for  lime. — The  lack  of  lime 
in  the  soils  of  humid  regions  is  so  universal  that  liming  will 
generally  increase  crop  growth.  For  example,  72  per  cent,  of 
the  soils  of  Pennsylvania  3  are  sour,  while  75  per  cent,  of  the 
cultivated  lands  of  Indiana4  show  acidity  by  the  ordinary 
tests.  While  it  is  safe  to  assume  that  the  productivity  of 
three-fourths  of  the  soils  in  the  eastern  part  of  the  United 
States  would  be  raised  by  liming,  it  is  a  question  in  many  cases 
whether  such  treatment  would  pay. 

1Ca(OH)2  +  C02  =  CaC03  +  H20. 
Mg(OH)2  4-  C02  =  MgCG3  +  H20. 
a  Kellogg,  J.  W.,  Lime  Beport;  Perm.  Dept.  Agr.,  Vol.  4,  No.  2,  Feb. 
1921. 

3  White,  J.  W.,  Lime  Bequirements  of  Pennsylvania  Soils;  Penn.  Agr. 
Exp.  Sta.,  Bui.  164,  1920. 

4  Wiancko,  A.  T.,  Conner,  S.  D.,  and  Jones,  S.  C,  The  Value  of  Lime  on 
Indiana  Soils;  Ind.  Agr.  Exp.  Sta.,  Bui.  213,  1918. 


366        NATUEE  AND  PROPERTIES  OF  SOILS 
Table  LXXI 


Foem  or  Lime 

Number 

op 
Samples 

CaO 

% 

MgO 

% 

Insolu- 
ble 

Matter 

Burned  lime  (low  mg.) . . . 
Burned  lime   (high  mg.).. 

Slaked  lime   (low  mg.) 

Slaked  lime    (high  mg.).. 

Pulverized  limestone 

Pulverized  oyster  shell .... 

Artificial  carbonate 

Marl 

59 
4 

242 
107 

161 

4 

72 

22 

70.01 
52.23 

64.26 

48.87 

47.83 
47.60 
50.70 
46.75 

2.97 
33.07 

3.73 
28.07 

3.19 

.85 

2.52 

1.00 

6.19 
2.81 

3.17 
1.58 

6.82 
8.78 
1.29 
5.90 

The  first  point  to  be  determined  in  deciding  whether  or 
not  lime  should  be  applied  is  in  regard  to  the  acidity  and  its 
degree.  The  litmus  or  zinc  sulfide  test  will  supply  this  in- 
formation, although  a  quantitative  determination  may  be 
made.1  The  general  degree  of  acidity,  unless  it  is  very  high, 
is  not  sufficient,  however,  in  deciding  whether  it  would  be 
wise  to  lime  the  soil.  The  nature  of  the  crops  is  a  .factor, 
as  well  as  the  type  of  the  rotation,  the  fertilizer  to  be  used, 
and  to  what  extent  farm  manure  and  green-crops  are  utilized. 
Often  special  considerations  are  involved,  such  as  scab  on 
potatoes,  which  is  encouraged  by  liming.  All  of  the  factors 
mentioned,  as  well  as  the  experiences  of  the  community  with 
lime,  should  be  considered  in  deciding  whether  liming  would 
pay.  If  the  increased  crops  that  will  probably  result  from 
an  application  of  lime  will  not  pay  a  good  interest  on  the 
investment,  then  liming  is  not  to  be  advised.  An  application 
sufficient  to  make  possible  the  production  of  good  crops  of 
clover  or  alfalfa  is  probably  all  that  can  be  used  profitably. 


1  The  teste  are  discussed  in  Chapter  XVIII. 


LIMING  THE  SOIL  367 

199.  Form  of  lime  to  apply. — The  experimental  data 
regarding  the  relative  effectiveness  of  the  different  forms  of 
lime  are  not  only  meagre  but  also  somewhat  contradictory. 
In  practice  it  is  best  to  assume  that  the  effectiveness  of  the 
lime  depends  on  the  amount  of  magnesium  and  calcium  car- 
ried and  is  influenced  to  a  much  less  degree  by  the  particular 
combinations  in  which  these  bases  may  occur.  For  example, 
one  and  a  half  tons  of  medium  to  finely  ground  limestone 
carrying  50  per  cent,  of  calcium  oxide  should  be  as  effective 
as  one  ton  of  burned  lime  analyzing  75  per  cent,  calcium  oxide. 
While  there  is  a  difference  in  the  rapidity  with  which  the 
various  forms  react,  there  seems  to  be  but  little  difference  be- 
tween them  over  the  period  of  a  rotation  when  they  are  ap- 
plied in  chemical  equivalent  amounts. 

Accepting  this  relationship  as  a  practical  working  basis, 
four  factors  must  be  considered  in  deciding  what  form  of 
agricultural  lime  to  apply.  These  factors  are  as  follows: 
(1)  chemical  equivalents,  determined  by  chemical  combina- 
tion and  purity;  (2)  cost  a  ton,  freight  on  board;  (3)  freight; 
and  (4)  cost  of  haul  and  application  to  the  land. 

It  is  evident  that,  if  the  various  forms  of  lime  are  equally 
effective  in  chemical  equivalent  quantities,  once  these  amounts 
are  determined  the  question  becomes  a  problem  in  arithmetic.1 
The  importance  of  the  factors  above  listed  can  best  be  shown 
by  working  out  an  actual  case.2 

^aO  x  1.32  =  Ca(OH)2  MgO  X  1.44  =  Mg(OH)2 

CaO  X  1.78  as  CaC08  MgO  X  '2.09  —  MgCO, 

Ca(OH)2  X    .76  =  CaO  Mg(OH)2  x    .69  =  MgO 

Ca(OH)2  X  1.35  as  CaCO,  Mg(OH)2  X  1.44  —  MgC03 
CaC03  X     .56  =  CaO  MgC03  X     .48  as  MgO 

CaC03X    .74  =  Ca(OH)3  MgC03  X     .69  =  Mg(OH)2 

CaO  X     .70  =  MgO  MgO  X  1.39  ss  CaO 

a  Calcium  oxide  and  calcium  hydroxide  have  an  advantage  over  ground 
limestone  in  percentages  of  calcium  carried  and  possibly  in  initial  ac- 
tivity. They  are,  however,  more  disagreeable  to  handle  and  do  not 
mix  with  the  soil  so  well  since  they  tend  to  lump  on  becoming  moist. 
Partially  or  wholly  carbonated  lumps  are  often  found  in  the  soil  years 
after  the  caustic  lime  has  been  applied. 


368        NATURE  AND  PROPERTIES  OF  SOILS 

Suppose  that  slaked  lime  carrying  70  per  cent,  of  calcium 
oxide  (CaO)  sells  in  carload  lots  at  $8.00  a  ton  and  that  pul- 
verized limestone  of  a  fair  degree  of  fineness  costs  in  bulk 
$4.50  and  analyzes  50  per  cent,  of  calcium  oxide.  Assume  the 
freight  as  $3.00  a  ton  and  the  cost  of  hauling  to  the  farm  and 
applying  to  the  land  as  $1.00  more. 

The  application  of  1  ton  of  the  agricultural  slaked  lime 
would  cost  $8.00  +  $3.00  +  $1.00  =  $12.00.  It  would  be 
necessary  to  apply  1.4  tons  of  the  limestone  to  every  ton  of 
slaked  lime.  This  would  amount  to  $6.30  +  $4.20  +  $1.40 
=  $11.90.  The  difference  in  this  case  is  very  slight  be- 
tween the  two  forms.  Lessening  the  freight  or  shortening 
the  haul  would  give  the  advantage  to  the  limestone,  while  in- 
creasing these  would  favor  the  use  of  slaked  lime. 

It  is  obvious  from  such  calculations  that  a  flat  recommenda- 
tion cannot  be  made  in  a  county  or  community  regarding  the 
lime  to  use.  Each  individual  case  should  be  calculated,  con- 
sidering the  cost  items  already  mentioned. 

200.  Amount  of  lime  to  apply. — The  possibility  of  an 
application  of  lime  paying  and  the  form  to  purchase  can  usu- 
ally be  determined  with  considerable  assurance.  Such  is  not 
the  case,  unfortunately,  regarding  the  amount  of  a  given  kind 
of  lime  to  apply  to  the  acre.  So  many  factors,  of  which  soil 
reaction  is  only  one,  are  active  in  determining  crop  growth 
that  acre  applications  are  at  best  estimates  and  often  admit- 
tedly guesses.  Not  only  the  degree  of  acidity  but  the  texture 
and  the  structure  of  the  soil,  the  crops  grown  in  rotation,  the 
length  of  the  rotation,  the  fertilizers  used,  the  amount  of  farm 
manure  added  in  a  given  period,  and  similar  conditions  must 
be  considered.  In  ordinary  practice,  it  is  seldom  economical 
to  apply  much  more  than  a  ton  of  limestone  or  its  equivalent 
to  the  acre,  unless  the  soil  is  very  acid  and  the  promise  for 
increased  crop  yield  exceptionally  good.  In  many  cases,  it 
seems  unnecessary  entirely  to  correct  the  acidity  of  a  soil  in 
order  to  promote  normal  crop  growth.    The  following  figures, 


LIMING  THE  SOIL 


369 


while  merely  tentative,  serve  in  a  general  way  as  guides  in 
practical  liming  operations  for  a  four-  or  five-year  rotation 
with  average  soils.  The  general  degree  of  acidity  may  be 
estimated  from  a  qualitative  test. 


Table  LXXXII 

SUGGESTED  AMOUNTS  OF  AVERAGE  PULVERIZED  LIMESTONE  THAT 

SHOULD  BE  APPLIED  TO  THE  ACRE  UNDER 

VARIOUS  CONDITIONS.1 


Acidity 

Limestone — Pounds  to  the  Acre 

Sandy  Loam 

Clay  Loam 

Moderate 

1200KL500 
Ii600h2500 

1800-2500 

Strong 

2500-3000 

201.  Changes  of  lime  in  the  soil. — When  calcium  oxide  or 
calcium  hydroxide  are  added  to  the  soil,  they  undergo  a  very 
rapid  transformation,  especially  if  the  soil  is  moist.  The 
oxide  takes  up  water  and  becomes  the  hydroxide,  while  the 
latter  almost  as  quickly  changes  to  the  carbonate.  The  reac- 
tions are  as  follows : 

CaO  +  H20  =  Ca(OH)2 
Ca(OH)2  +  C02  =  CaC03  +  H20 

It  is  generally  supposed  that  when  once  the  carbonate  is 
formed  in  the  soil  or  added  as  pulverized  limestone,  it  is  more 

1  The  equivalent  amounts   of  burned  or  slaked  lime  may  readily  be 

calculated  from  the  chemical  equivalents  already  quoted.     Calculate  for 

example  the  amount  of  slaked  lime,  carrying  65  per  cent,  of  CaO  and 

5  per  cent,  of  MgO,  necessary  to  equal  an  application  of  2000  pounds  of 

adequately  pulverized  limestone  containing  48  per  cent,  of  CaO  and  2 

per  cent,  of  MgO.     The  5  per  cent,  of  MgO  in  the  slaked  lime  and  the 

2  per  cent,  of  MgO  in  the  limestone  are  equivalent  in  neutralizing  capacity 

to  6.9  and  2.8  per  cent,  of  CaO,  respectively.    The  slaked  lime  and  the 

limestone,  therefore,  carry  the  equivalent  of  71.9  and  50.8  per  cent,  of 

2000  X  508 
CaO,  respectively.     =-^ ==  1413   pounds,  the  amount  of  slaked 

lime  necessary  to  equal  2000  pounds  of  the  limestone. 


370        NATURE  AND  PROPERTIES  OF  SOILS 

or  less  stable,  except  for  slow  solubility.  In  most  cases,  how- 
ever, the  carbonate,  especially  magnesium  carbonate,  is  rap- 
idly decomposed  and  carbon  dioxide  is  given  off,  the  bases 
presumably  entering  the  unsaturated  aluminum  silicates 
which  are  likely  to  be  present  in  acid  soils.1 

The  actual  loss  of  lime  in  drainage  water  occurs  through 
the  influence  of  carbon  dioxide  which  changes  the  insoluble 
carbonate  to  the  soluble  bicarbonate.  The  bicarbonate  is 
washed  out  as  such  or  ionizes,  the  calcium  and  the  magnesium 
being  lost  in  the  ionic  state.  The  presence  of  nitrates  in  the 
soil,  either  from  biological  activity  or  from  fertilizers,  also 
greatly  facilitates  the  loss  of  lime  from  the  soil  in  drainage. 
Such  influence  is  to  be  especially  expected  during  the  summer 
and  fall.  In  spite  of  the  direct  effect  of  carbon  dioxide  and 
nitrates  on  the  loss  of  lime,  the  controlling  factor  seems  to  be 
the  amount  of  water  passing  through  the  soil  rather  than  its 
concentration.  The  following  unpublished  data  from  the 
Cornell  University  lysimeters  show  the  losses  of  lime  that  may 
be  expected  under  different  conditions.2  These  figures  are 
averages  of  ten  years '  work  with  Dunkirk  silty  clay  loam. 

Table  LXXXIII 

AVERAGE   ANNUAL   LOSS   OF   NITROGEN    AND   LIME   BY   LEACHING. 
CORNELL  LYSIMETERS.      AVERAGE  OF  10  YEARS. 


Pounds  to  the  Acre  Per  Year 

Condition 

Nitrogen 

LIME  ex- 
pressed AS 
CaO 

LIME  EX- 
PRESSED AS 

CaCO, 

Bare  soil 

69.0 
7.3 
2.5 

557.0 
345.9 
363.8 

993  6 

Rotation 

6171 

Grass 

648.9 

1  Maclntire,   et   al.,   The  Non-existence  of  Magnesium   Carbonate  in 
Humid  Soils;  Tenn.  Agr.  Exp.  Sta.,  Bui.  107,  1914. 

2  Complete  data  on  these  lysimeters  will  be  found  in  par.  163. 


LIMING  THE  SOIL  371 

202.  Effect  of  lime  on  the  soil. — In  heavy  soils  there  is 
always  a  tendency  for  the  fine  particles  to  become  too  closely 
associated.  Such  a  condition  interferes  with  air  and  water 
movement.  The  granular  structure  that  should  prevail  is 
somewhat  encouraged  by  the  addition  of  lime,  especially  the 
caustic  forms.  In  practice,  however,  the  amounts  of  lime  ap- 
plied are  generally  too  small  to  have  much  importance  in  this 
respect. 

Chemically,  lime  brings  about  many  complex  changes  in 
the  soil.  Basic  exchange  is  forced  and  certain  mineral  nu- 
trients tend  to  become  more  available.  The  hydrogen  ion 
concentration  is  lowered  and  deleterious  bases,  such  as  alumi- 
num and  manganese,  are  forced  back  into  less  active  combi- 
nations. Oxidation  processes  seem  also  to  be  stimulated,  thus 
favoring  the  elimination  of  organic  toxins,  which  often  de- 
velop when  improper  decay  takes  place.  The  charge  that 
quicklime  in  normal  amounts  produces  a  rapid  and  detri- 
mental oxidation  of  the  soil  organic  matter  is  probably  an 
over-statement.1  While  lime  of  all  kinds  promotes  the  oxida- 
tion of  organic  matter,  calcium  oxide,  when  added  in  rational 
amounts,  is  probably  no  more  active  over  the  term  of  the  rota- 
tion than  calcium  carbonate. 

Most  of  the  favorable  soil  organisms  and  some  of  the  un- 
favorable ones,  such  as  those  that  produce  potato-scab,  are 
benefited  by  judicious  liming.  The  bacteria  that  fix  nitrogen 
from  the  air,  either  alone  or  in  the  nodules  of  some  legumes, 
are  especially  stimulated  by  the  application  of  lime.  The 
change  of  ammoniacal  nitrogen  to  the  nitrate  form,  which  is  a 
biological  phenomenon,  requires  active  basic  material.  Other- 
wise this  necessary  transformation  will  not  proceed.  The 
decomposition  of  both  carbohydrate  compounds  (fermenta- 
tion) and  of  nitrogenous  materials  (putrefaction)  depends  on 
lime,  that  the  decay  products  may  be  favorable. 

1Madntire,  W.  H.,  The  Carbonation  of  Burned  Lime  in  Soils;  Soil 
Sci.,  Vol.  VII,  No.  5,  pp.  325-446,  May,  1919. 


372        NATURE  AND  PROPERTIES  OF  SOILS 

Of  the  general  and  specific  influences  of  lime  just  men- 
tioned the  correction  of  acidity  is  the  one  commonly  ascribed 
to  it  in  the  popular  mind.  The  mere  correction  of  the  soil 
reaction,  however,  is  probably  no  more  important  than  a 
number  of  other  direct  and  indirect  influences  of  lime.  It  is 
evident  that  the  benefits  that  may  result  from  liming  a  soil 
will  accrue  from  a  combination  of  influences  rather  than  from 
one  effect  alone. 

203.  Crop  response  to  liming. — Much  experimental  work 
has  been  done  in  various  parts  of  the  world  in  determining 
the  relative  response  of  different  crops  to  liming  and  the  rea- 
son for  certain  well-known  differences.  As  might  be  expected, 
the  results,  while  in  close  agreement  as  to  some  crops,  show 
striking  disagreements  as  to  others.  This  is  to  be  expected, 
since  the  varying  conditions  of  the  experiments  would  have  a 
marked  influence  on  the  response  of  the  plants  under  con- 
sideration. 

Of  legume  crops,  alfalfa  and  red  and  white  clovers  respond 
most  markedly  to  lime.  The  response  of  soybeans,  garden 
peas  and  field  peas,  while  less,  is  still  quite  noticeable.  Alsike 
clover  is  more  tolerant  to  acidity  than  red  clover  and,  as  the 
soil  of  a  region  declines  in  active  bases,  it  is  common  to  find 
it  gradually  replacing  the  latter.  Japanese  clover,  cowpeas, 
vetch,  and  field  beans  do  not  seem  to  be  greatly  benefited  by 
lime. 

Of  the  non-legumes  that  are  favorably  influenced  by  lime, 
blue-grass,  maize,  timothy,  oats,  barley,  wheat,  and  sorghum 
may  be  mentioned.  Rye  is  less  benefited  by  liming  than  is 
barley.  Red-top,  cotton,  strawberries,  and  potatoes  do  not 
seem  to  be  particularly  stimulated  by  liming.  Certain  plants, 
such  as  blueberries,  watermelons,  and  rhododendron  are  ac- 
tually injured  by  the  use  of  lime. 

.  There  are  a  number  of  reasons  why  plants  may  be  benefited 
by  lime,  these  reasons  being  numerous  and  complex  enough 
to  account  for  the  differences  in  response  among  common 


LIMING  THE  SOIL  373 

crops.  The  possible  influences  of  lime  on  plants  may  be  listed 
as  follows:  (1)  direct  nutritive  action;  (2)  synergistic  rela- 
tionships either  in  the  soil  solution  or  in  the  cell- wall;  (3)  re- 
moval or  neutralization  of  toxins  of  either  an  organic  or  inor- 
ganic nature;  (4)  effect  on  plant  diseases;  (5)  liberation  of 
mineral  nutrients;  and  (6)  encouragement  of  the  biological 
preparation  of  nutrient  materials. 

In  some  cases  the  calcium  may  function  as  a  direct  nutrient ; 
in  others  the  intake  of  nutrients  may  be  facilitated  by  the 
presence  of  calcium  and  magnesium ;  while  in  still  other  cases 
the  elimination  or  alleviation  of  a  toxic  condition  may  be  the 
important  result.  It  is  easy  to  conceive  that  any  two  or  all 
three  of  these  relationships  might  be  fulfilled  simultaneously 
by  lime.  The  stimulating  influence  of  lime  might  also  make 
the  plant  a  more  active  agent  and  thus  encourage  it  to  aid 
to  a  greater  extent  in  the  preparation  of  its  own  nutrients. 
Certain  diseases  may  be  retarded  or  even  entirely  suppressed 
by  lime,  as  is  the  ' '  finger-and-toe ' '  disease  of  the  Cruciferae. 

The  liberation  of  mineral  nutrients,  such  as  potash  and 
phosphoric  acid,  by  the  addition  of  lime,  is  somewhat  uncer- 
tain although  it  evidently  does  occur  in  many  cases.1  The 
process  is  probably  a  more  or  less  complicated  physical  or 
chemical  change.  The  stimulation  to  plants  by  such  an  ac- 
tion is  difficult  to  establish,  since  so  many  disturbing  factors 
are  active  in  obscuring  the  results.  Lime  is  undoubtedly  very 
important  in  the  use  of  acid  phosphate,  the  active  compound 
of  which  is  mono-calcium  phosphate  (CaH4(P04)2).  In  the 
presence  of  active  calcium,  the  reversion  compound  is 
(Ca3(P04)2),2  rather  than  the  very  insoluble  iron  and  alumi- 
num phosphates  (FeP04  and  A1P04). 

The  formation  of  nitrates  proceeds  rather  slowly  in  most 

Glummer,  J.  K.,  The  Effects  of  Liming1  on  the  Availability  of  Soil 
Potassium,  Phosphorus  and  Sulfur;  Jour.  Amer.  Soc.  Agron.,  Vol.  13, 
No.  4,  pp.  162-171,  1921. 

2CaH4(P04)2  + 2CaH2(C03)2  =  Ca3(P04)2-f-4H20  +  4COa 


374        NATURE  AND  PROPERTIES  OF  SOILS 

acid  soils,  since  there  is  but  little  active  basic  material  to 
stimulate  the  nitrifying  organisms  directly  or  to  neutralize 
the  nitrous  acid  that  is  formed.1  The  addition  of  lime  is  the 
most  economical  method  of  supplying  this  base.  This  response 
of  the  nitrifying  bacteria  to  lime  is  a  matter  of  great  moment 
to  crops  that  need  large  amounts  of  nitrate  nitrogen  and  may 
account  in  some  cases  for  the  early  response  of  certain  crops 
to  liming.  The  tolerance  of  some  plants  to  acid  soils  might  be 
accounted  for  on  the  supposition  that  they  need  but  small 
amounts  of  nitrogen  or  are  able  to  absorb  their  nitrogen  in 
forms  other  than  the  nitrate. 

204.  Method  and  time  of  applying  the  lime. — Although 
lime  is  lost  rapidly  from  most  soils,  appearing  in  the  drain- 
age water  in  large  amounts,  it  does  not  seem  to  correct  to  any 
great  extent  the  acidity  of  the  soil  layers  through  which  it  is 
carried.2  Lime  applied  at  the  soil  surface  will  tend  to  disap- 
pear, but  will  have  little  effect  on  the  soil  below.  The  action 
of  lime  seems  to  be  a  contact  phenomenon  and  the  more  thor- 
oughly it  is  mixed  with  the  soil,  the  greater  will  be  the  num- 
ber of  active  focii  and  the  more  rapid  and  effective  will  be  the 
results  of  the  treatment. 

Lime  Is  best  applied  to  plowed  land  and  worked  into  the  soil 
as  the  seed-bed  is  prepared.  It  should  be  thoroughly  mixed 
with  the  surface  three  to  five  inches  of  soil.  Top-dressing  of 
lime  is  seldom  recommended  except  on  permanent  meadows 
and  pastures.  The  time  of  year  at  which  lime  is  applied  is 
immaterial,  the  system  of  farming,  the  type  of  rotation,  and 
such  considerations  being  the  deciding  factors.  The  soil 
should  not  be  too  moist  when  the  application  is  made,  as  the 

1 2NH,  +  30,  =  2HNO,  +  2H20. 
2HN03  +  CaCO,  =  Ca(N02),  +  H20  +  CO,. 
Ca(N02),  +  02  =  Ca(NO,),. 
2  Wilson,  B.  D.,  The  Translocation  of  Calcium  in  a  Soil;  Cornell  Agr. 
Exp,  Sta.,  Memoir  17,  1918. 

Stewart,  E.,  and  Wyatt,  F.  A.,  Limestone  Action  on  Acid  Soils;  I1L 
Agr.  Exp.  Sta.,  Bui.  212,  1919. 


LIMING  THE  SOIL  375 

lime,  especially  the  slaked  and  ground  burned  forms,  tends  to 
ball  badly  and  thus  thorough  distribution  is  prevented. 

A  lime  distributer  should  be  used,  especially  if  the  amount 
to  be  applied  is  at  all  large.  A  manure-spreader  can  be  util- 
ized and  even  an  end-gate  seeder  may  be  pressed  into  service. 
Small  amounts  of  lime  may  be  distributed  by  means  of  the 
fertilizer  attachment  on  a  grain  drill.  As  with  the  applica- 
tion of  any  material,  the  evenness  of  distribution  is  as  im- 
portant as  the  form  and  amount  of  lime  used  and  should  by  no 
means  be  neglected. 

A  discussion  of  the  application  of  lime  is  never  complete 
without  some  consideration  being  given  to  the  place  in  the 
rotation  at  which  the  liming  is  best  done.  In  a  rotation  of 
maize,  oats,  wheat,  and  two  years  of  clover  and  timothy,  the 
lime  is  often  applied  when  the  wheat  is  seeded  in  the  fall.  It 
can  then  be  spread  on  the  plowed  ground  and  worked  in  as 
the  seed-bed  is  prepared.  Its  effect  is  thus  especially  favor- 
able on  the  new  seeding.  Thorne  x  has  shown,  however,  in 
certain  Ohio  experiments,  that  maize  is  affected  more  favor- 
ably than  any  of  the  crops  above  mentioned  and  as  the  money 
value  of  this  increase  is  practically  as  much  as  that  from  the 
hay,  he  favors  applying  the  lime  to  the  maize.  With  pota- 
toes in  the  rotation,  the  lime  should  follow  the  potato  crop, 
especially  if  scab  is  prevalent.  In  practice  the  place  of  lime 
in  the  rotation  is  usually  determined  by  expediency,  since  the 
vital  consideration  is,  after  all,  the  application  of  lime  regu- 
larly and  in  conjunction  with  a  rational  rotation  of  some  kind. 

205.  The  calcium  and  magnesium  ratio. — A  physiological 
balance  seems  to  be  necessary  in  a  nutrient  solution  in  con- 
tact with  a  normally  growing  plant.  This  balance  varies  with 
the  plant  and  with  numerous  other  conditions.  The  reason 
for  such  antagonistic  action  between  the  ions  of  certain  ele- 
ments is  difficult  to  explain  and  many  theories  have  been  ad- 

1  Thorne,  C.  E.,  The  Maintenance  of  Fertility.  Liming  the  Land; 
Ohio  Agr.  Exp.  Sta.,  Bui.  279,  1914. 


376        NATURE  AND  PROPERTIES  OF  SOILS 

vanced.  Loew,1  in  1901,  worked  out  the  optimum  ratio  for 
a  number  of  different  plants  growing  in  water  culture.  He 
found  that  both  calcium  and  magnesium  alone  were  toxic  and 
it  was  only  when  the  ratio  of  these  ions  fell  within  certain 
limits  that  the  toxicity  disappeared.  This  ratio  varied  be- 
tween 1  of  CaO  to  1  of  MgO  and  7  of  CaO  to  1  of  MgO. 

The  question  was  immediately  raised  as  to  the  advisability 
of  using  limestone  or  even  burned  and  slaked  lime,  the  mag- 
nesium content  of  which  approached  in  any  degree  the  cal- 
cium present.  Recent  field  and  laboratory  tests  have  shown, 
however,  that  magnesium  salts  may  be  applied  in  ordinary 
amounts  alone  or  with  calcium  compounds  with  impunity.2 
The  absorptive  capacity  of  the  soil  seems  to  take  care  in  a 
very  effective  way  of  any  toxicity  that  might  result  from  a 
soil  solution  physiologically  unbalanced. 

206.  The  fineness  of  limestone. — The  hardness  of  the 
stone,  its  purity,  and  its  fineness  are  items  of  extreme  im- 
portance to  the  manufacturer  of  pulverized  lime.  The  softer 
the  limestone,  the  easier  the  grinding  and  the  finer  the  product 
with  a  given  expenditure  of  power.  The  higher  the  percent- 
age of  calcium  and  magnesium,  the  greater  is  the  effectiveness 
of  a  given  quantity.  The  farmer,  other  conditions  being  more 
or  less  equal,  is  especially  interested  in  the  fineness  of  the 
product.  It  is  a  well-known  fact  that  the  finer  the  division  of 
any  material,  the  more  rapid  the  solution.     This,  however, 

1Loew,  O.,  The  Physiological  Bole  of  the  Mineral  Nutrients  of  Plants; 
U.  S.  Dept.  Agr.,  Bur.  Plant  Ind.,  Bui.  1,  p.  53,  1901. 

3  Gile,  P.  L.,  and  Ageton,  C.  U.,  The  Significance  of  the  Lime-Mag- 
nesia Ratio  in  Soil  Analyses;  Jour.  Ind.  and  Eng.  Chem.,  Vol.  5,  pp. 
33-35,  1913. 

Thomas,  W.,  and  Frear,  W.,  The  Lime-Magnesia  Batio  in  Soil  Amend- 
ments; Jour.  Ind.  and  Eng.  Chem.,  Vol.  7,  No.  12,  pp.  1042-1044, 
Dec.  1915. 

Lipman,  C.  B.,  A  Critique  of  the  Hypothesis  of  the  Lime-Magnesia 
Batio;  Plant  World,  Vol.  19,  No.  4,  pp.  83-105,  Apr.  1916. 

Wyatt,  F.  A.,  Influence  of  Calcium  and  Magnesium  Compounds  on 
Plant  Growth;  Jour.  Agr.  Kes.,  Vol.  VI,  No.  16,  pp.  589-619;  1916. 

Stewart,  B.,  and  Wyatt,  F.  A.,  Limestone  Action  on  Acid  Soils;  111. 
Agr.  Exp.  Sta.,  Bui.  212,  1919. 


LIMING  THE  SOIL 


377 


is  not  the  only  importance  of  fineness.  Lime  produces  its  in- 
fluence largely  through  contact,  and  the  finer  the  lime  is 
ground,  the  more  thorough  is  the  mixing  with  the  soil  and 
the  greater  the  number  of  operating  focii. 

White  x  presents  the  following  significant  data  as  a  result 
of  certain  laboratory  and  greenhouse  studies  at  State  College, 
Pennsylvania. 

Table  LXXXIV 

A   COMPARISON    OF    VARIOUS   GRADES2    OF   LIMESTONE   WHEN 
APPLIED  AT  THE  SAME  RATES. 


Conditions 


Solubility  in  carbonated  water. 
Value  in  correcting  acidity .... 

Formation  of  nitrates 

Plant  growth 


100  Mesh 

60-80 

20-40 

8-12 

AND 

Smaller 

Mesh 

Mesh 

Mesh 

100 

57 

45 

28 

100 

57 

27 

18 

100 

94 

56 

12 

100 

69 

22 

5 

These  figures  show  that  the  finer  grades  of  limestone  are 
much  more  rapidly  effective.    Further  data  by  the  same  au- 

1  White,  J.  W.,  The  Value  of  Limestone  of  Different  Degrees  of  Fine- 
ness; Penn.  Agr.  Exp.  Sta.,  Bui.  149,  1917.  Also,  Thomas,  W.,  and 
Frear,  W.,  The  Importance  of  Fineness  of  Sub-division  to  the  Utility  of 
Crushed  Limestone  as  a  Soil  Amendment;  Jour.  Ind.  and  Eng.  Chem., 
Vol.  7,  No.  12,  pp.  1041-1042,  1915. 

Broughton,  L.  B.,  et  al,  Tests  of  the  Availability  of  Different  Grades 
of  Ground  Limestone ;  Md.  Agr.  Exp.  Sta.,  Bui.  193,  1916. 

Kopeloff,  N.,  The  Influence  of  Fineness  of  Division  of  Pulverized 
Limestone  on  Crop  Yield  as  Well  as  the  Chemical  and  Bacteriological 
Factors  in  Soil  Fertility;  Soil  Sci.,  Vol.  IV,  No.  1,  pp.  19-67,  1917. 

Frear,  W.,  The  Fineness  of  Lime  and  Limestone  Application  as  Be- 
lated to  Crop  Production;  Jour.  Amer.  Soc.  Agron.,  Vol.  13,  No.  4, 
pp.  171-174,  1921. 

2  Lime  is  graded  by  sieves  carrying  a  certain  number  of  meshes  to  the 
linear  inch.  An  80-mesh  sieve  has  80  openings  to  the  linear  inch  or  6400 
to  the  square  inch.  Screens  rated  as  carrying  the  same  number  of  meshes 
often  do  not  give  the  same  grade  of  material,  due  to  a  difference  in  the 
size  of  wire  used.  Material  of  60  to  80  mesh  refers  to  those  sizes  that 
will  pass  through  a  60-mesh  but  will  be  held  by  an  80-mesh  screen. 
A  standardization  of  sieves  and  methods  of  expressing  such  analyses  is 
much  needed. 


378         NATURE  AND  PROPERTIES  OF  SOILS 

thor  indicate  that  while  the  coarser  lime  is  less  rapid  in  its 
action,  it  remains  in  the  soil  longer  and  its  influence  should 
be  effective  for  a  greater  period  of  years. 


Table  LXXXV 

DECOMPOSITION  OF  LIMESTONE  DURING  THE  THREE  YEARS 
AFTER  APPLICATION. 


Percentage  of 

Decomposition 

Mesh 

High  Calcium 
Stone 

High  Magnesium 
Stone 

100  mesh  and  smaller.  . . 
60  to  80  mesh 

92.4 
81.5 
46.7 
14.9 

91.2 
72.2 

20  to  40  mesh 

34.9 

8  to  12  mesh 

5.9 

The  conclusion  is  likely  to  be  drawn  that  limestone  should 
be  ground  as  finely  as  possible.  Such  an  assumption  is  at 
fault  in  several  ways.  In  the  first  place,  very  fine  lime  is 
difficult  to  handle  and  unpleasant  to  distribute.  Again,  the 
cost  of  grinding  increases  very  rapidly  with  the  fineness,  being 
entirely  too  expensive  compared  with  the  results  attained. 
Moreover,  finely  ground  material  does  not  possess  the  lasting 
qualities  of  the  coarser  lime.  Because  of  the  cost  of  grinding 
the  stone  to  a  very  fine  condition  and  the  rapidity  with  which 
such  material  disappears  from  the  soil,  a  medium  ground 
lime  seems  to  be  a  more  desirable  commercial  product.  Such 
material  has  enough  of  the  finer  particles  to  give  quick  re- 
sults and  yet  enough  of  the  coarser  fragments  to  make  it  last 
over  the  period  of  the  rotation.  A  pulverized  limestone,  all 
of  which  will  pass  a  10-mesh  sieve,  70  per  cent,  of  which 
will  pass  a  50-mesh  sieve  and  50  per  cent,  of  which  will  pass 
a  100-mesh  sieve,  should  give  excellent  results  and  yet  be 
cheap  enough  to  make  its  use  worth  while. 

The  following  figures  show  in  an  approximate  way  the 


LIMING  THE  SOIL 


379 


mechanical  composition  of  limestone  on  sale  in  Pennsylvania 
for  19201: 

Table  LXXXVI 

MECHANICAL    COMPOSITION    OF    SOME    LIMESTONE    OFFERED    FOR 
SALE  IN  PENNSYLVANIA  IN  1920. 


Limestone 

Amount  Passing  Sieve,  Mesh 

10 

50 

100 

1    

100 
100 
100 
100 
100 
100 

98 
99 
89 
70 
57 
44 

92 

2   

88 

3   

73 

4   

58 

5   

50 

6   

34 

207.  Gypsum  and  other  soil  amendments. — Gypsum,  in 
which  form  calcium  sulfate  (CaS04.2H20)  is  usually  applied 
to  soil,  has  been  used  for  years  and  was  popular  long  before 
commercial  fertilizers  were  available  to  any  extent.  The  use 
of  gypsum  was  probably  familiar  to  the  Romans.  It  fre- 
quently goes  by  the  name  land  plaster.  It  is  widely  distribu- 
ted in  nature  and  easily  ground.  Its  beneficial  effect  has  been 
noted,  particularly  with  clover  and  alfalfa,  crops  which  re- 
spond especially  to  potash.  Its  popularity  has  waned  in  recent 
years,  however,  since  its  effectiveness  on  soils  where  it  has 
long  been  used  has  apparently  decreased.  This  possibly  has 
been  due  in  part  to  the  acid  residue  that  ultimately  must  re- 
sult from  the  use  of  such  material  and  to  the  failure  to  lib- 
erate potassium — a  property  with  which  it  has  very  gen- 
erally been  credited  and  which,  when  applied  to  some  soils, 
it  may  possess.  The  experimental  work  in  this  respect  is 
somewhat  conflicting,  possibly  due  to  the  fact  that  the  con- 

1  Kellogg,  J.  W.,  Lime  Beport;  Penn.  Dept.  Agr.,  Vol.  4,  No.  2, 
1921. 


380        NATURE  AND  PROPERTIES  OF  SOILS 

ditions  of  contact  between  the  soil  and  the  gypsum  were  ab- 
normal. McMillar *  found  that  the  potash  of  certain  Minne- 
sota soils  treated  with  one  per  cent,  of  gypsum  was  appre- 
ciably influenced  three  months  after  the  application.  When 
gypsum  has  proven  beneficial  to  crop  growth,  the  effect  may 
have  been  due  to  the  nutrient  influence  of  the  sulfur  it  con- 
tains or  to  the  potash  liberated  from  its  soil  combinations. 
The  use  of  gypsum  as  a  soil  amendment  is  now  seldom  recom- 
mended, especially  if  the  other  forms  of  lime  are  available. 

Sodium  chloride  has  a  marked  effect  on  the  productivity  of 
some  soils,  especially  when  certain  crops  such  as  asparagus 
are  grown.  Wherein  its  effectiveness  lies  is  not  well  under- 
stood. Increased  fertility  arising  from  the  addition  of  sodium 
and  chlorine,  which  are  plant  constituents,  is  probably  not 
the  reason  of  its  influence,  as  these  substances  are  usually 
available  in  soils  far  beyond  any  possible  plant  requirement. 
When  common  salt  shows  a  beneficial  influence,  it  is  probably 
due  to  its  tendency  to  liberate  certain  mineral  nutrients  such 
as  potassium,  calcium,  and  magnesium.  Since  it  tends  to 
leave  an  acid  residue  in  the  soil  and  since  some  form  of  lime 
will  generally  give  better  and  more  permanent  results,  the 
use  of  common  salt  is  not  recommended  except  in  certain 
cases. 

The  use  of  di-calcium  silicate  (Ca2Si04)  in  an  experimental 
way  as  a  liming  material  has  recently  received  some  attention. 
Cowles,2  in  1917,  presented  data  from  which  he  concluded  that 

1  McMillar,  P.  R.,  Influence  of  Gypsum  upon  the  Solubility  of  Potash 
in  Soils;  Jour.  Agr.  Res.,  Vol.  XIV,  No.  1,  pp.  61-66,  1918. 

Morse,  F.  W.,  and  Curry,  B.  E.,  The  Availability  of  Soil  Potash  in 
Clay  and  Clay  Loam  Soils;  N.  H.  Agr.  Exp.  Sta.,  Bui.  142,  1909. 

Bradley,  C.  E.,  The  Eeaction  of  Lime  and  Gypsum  on  Some  Oregon 
Soils;  Jour.  Ind.  and  Eng.  Chem.,  Vol.  2,  No.  12,  pp.  529-530,  1910. 

Briggs,  L.  J.,  and  Breazeale,  J.  F.,  Availability  of  Potash  in  Certain 
Orthoclase-b earing  Soils  as  Affected  by  Lime  or  Gypsum;  Jour.  Agr. 
Res.,  Vol.  VIII,  No.  1,  pp.  21-28,  1917. 

a  Cowles,  A.  H.,  Calcium  Silicates  as  Fertilizers.  Metal.  Chem.  Eng., 
Vol.  17,  pp.  664-665,  1917. 


LIMING  THE  SOIL  381 

this  compound  was  of  greater  value  than  either  ground  lime- 
stone or  slaked  lime  as  an  amendment.  He  also  concluded 
that  silicon  was  an  essential  element  in  plant  nutrition.  Hart- 
well  and  Pember,1  in  1920,  found  di-calcium  silicate  approxi- 
mately equal  to  limestone  insofar  as  the  correction  of  acidity 
was  concerned.  Lettuce  was  used  as  an  indicator.  They 
found  no  indication  that  the  silicon  was  of  any  value,  but,  as 
their  experiments  were  with  soil,  this,  of  course,  does  not  op- 
pose the  idea  that  silicon  is  an  essential  element  in  the  growth 
of  plants. 

Hartwell  and  Pember  concluded  that  the  beneficial  influ- 
ence of  phosphorus  and  calcium  compounds  added  to  the  soil 
might,  in  many  cases,  be  due  to  the  precipitation  of  active 
aluminum  quite  as  much  as  to  the  supplying  of  nutrients  or 
the  correction  of  actual  acidity.  Such  a  conception  of  the 
influence  of  liming  materials  may  ultimately  mean  an  in- 
crease in  the  number  and  nature  of  the  compounds  that  may 
be  used  as  soil  amendments. 

208.  Importance  of  lime  in  soil  improvement.2 — The  in- 
fluence of  successively  liming  a  soil  over  a  period  of  years 
may  tend  to  raise  or  lower  the  fertility  of  the  soil,  according 
to  the  system  of  soil  management  that  accompanies  the  appli- 
cations of  the  lime.  The  use  of  lime  alone  will  undoubtedly 
increase  crop  yield  for  a  time.     Basic  exchange  will  be  en- 

1  Hartwell,  B.  L.,  and  Pember,  F.  E.,  The  Effect  of  Dicalcium  Silicate 
on  an  Acid  Soil;  Soil  Sci.,  Vol.  X,  No.  1,  pp.  57-60,  July,  1920. 

2  A  number  of  general  references  on  the  importance  of  lime  were 
given  at  the  beginning  of  the  chapter.    See  also, 

Wiancko,  A.  T.,  et  al.,  The  Value  of  Lime  on  Indiana  Soils;  Ind.  Agr. 
Exp.  Sta.,  Bui.  213,  1918. 

Stewart,  R.,  and  Wyatt,  P.  A.,  Limestone  Action  on  Acid  Soils;  111. 
Agr.  Exp.  Sta.,  Bui.  212,  1919. 

Lipman,  J.  G.,  and  Blair,  A.  W.,  The  Lime  Factor  in  Permanent 
Soil  Improvement;  Soil  Sci.,  Vol.  IX,  No.  2,  pp.  83-114,  Feb.  1920. 

Hartwell,  B.  L.,  and  Damon,  S.  C,  Six  Years'  Experience  in  Improving 
a  Light  Unproductive  Soil;  Jour.  Amer.  Soc.  Agron.,  Vol.  13s  No.  1, 
pp.  37-41,  Jan.  1921. 


382         NATURE  AND  PROPERTIES  OF  SOILS 

couraged,  soil  bacteria  will  be  stimulated,  and  more  nutrients 
will  become  available  for  crop  use.  Such  stimulation,  how- 
ever, will  soon  wane,  and  if  nothing  is  returned  to  the  land, 
productivity  must  ultimately  drop  back  to  even  a  lower  level 
than  before  the  lime  was  applied. 

Lime  is,  to  a  great  extent,  a  soil  amendment  and  as  it  in- 
creases crop  growth,  the  draft  on  the  soil  becomes  larger. 
Greater  effort  is  necessary,  therefore,  in  order  to  maintain 
the  fertility  of  the  land  when  lime  is  used  than  when  such  ap- 
plications are  not  made.  Farm  manure,  crop  residues  and 
green-manures  should  be  utilized  to  the  fullest  extent  and 
when  these  are  insufficient  to  keep  up  the  potash  and  phos- 
phoric acid  of  the  soil,  commercial  fertilizing  materials  must 
be  resorted  to.  Lime  improperly  used  exhausts  the  soil,  but 
when  properly  and  rationally  applied  it  becomes  one  of  the 
important  factors  in  the  maintenance  of  a  more  or  less  con- 
tinuous productivity. 

It  is  interesting  in  this  connection  to  consider  certain  fig- 
ures from  the  Ohio  Experiment  Station.1  Maize,  oats,  wheat 
and  clover  and  timothy  were  grown  in  a  five-year  rotation 
on  both  limed  and  unlimed  plats  fertilized  in  various  ways. 
The  results  of  table  LXXXVII  (page  383)  are  averages  for  a 
period  of  twelve  years. 

It  is  immediately  evident  that  the  effectiveness  of  the  lime 
was  increased  by  the  use  of  both  fertilizers  and  farm  manure. 
Conversely,  the  returns  from  the  fertilizers  and  the  manure 
were  markedly  influenced  by  the  lime.  The  lime  increased 
the  effectiveness  of  the  acid  phosphate  20  per  cent.  The  in- 
creases with  the  acid  phosphate  plus  potassium  chloride  and 
with  the  complete  fertilizers  were  22  and  10  per  cent.,  re- 
spectively. Lime  increased  the  returns  of  farm  manure  only 
4  per  cent.,  indicating  that  manure  itself  may  function  as  a 

1  Thome,  C.  E.,  The  Maintenance  of  Soil  Fertility.  Liming  the  Land; 
Ohio  Agr.  Exp.  Sta.,  Bui.  279,  1914. 


LIMING  THE  SOIL 


383 


Table  LXXXVII 

RELATIVE  ROTATION  VALUES  OF   CROP  INCREASES  DUE  TO  LIMING 
AND  FERTILIZING   A   STANDARD   ROTATION   OVER  A   TWELVE- 
YEAR  PERIOD.    OHIO  EXPERIMENT  STATION.     THE  ACID 
PHOSPHATE    TREATMENT    IS    TAKEN    AS    100   FOR 
THE  LIME  GAIN   AND   ALSO  FOR   THE 
UNLIMED  FERTILIZER   GAIN. 


Fertilizers  to  the  Dotation 

Gain 
from 
Lime 

Gain  from  Fer- 
tilizers 

Unlimed 

Limed 

Acid  phosphate 

100 

114 

119 
113 

100 
142 

232 

287 

120 

Acid  phosphate  plus  potassium 
chloride 

173 

Acid  phosphate,  potassium 

chloride  and  sodium  nitrate 

Manure,  16  tons 

255 
300 

soil  amendment.  These  figures  serve  in  a  definite  way  to  em- 
phasize the  correlation  between  liming  and  the  other  factors 
that  must  be  considered  in  soil  improvement  and  fertility 
maintenance. 


CHAPTER  XX 

SOIL   ORGANISMS,1   CARBON,  SULFUR,  AND 
MINERAL  CYCLES 

A  vast  number  of  organisms,  both  vegetable  and  animal, 
live  in  the  upper  layers  of  the  soil  and  determine  to  a  very 
large  degree  its  dynamic  character.2  By  far  the  greater  por- 
tion of  these  organisms  belong  to  plant  life,  producing  those 
changes,  both  organic  and  inorganic,  which  control,  in  large 
degree,  the  productivity  of  the  soil.  While  most  of  the  or- 
ganisms are  so  minute  as  to  be  seen,  if  visible  at  all,  only  by 
the  aid  of  a  microscope,  a  small  proportion  attain  the  size  of 
the  larger  rodents.  For  convenience  of  discussion  the  life  of 
the  soil  may  be  classified  into  macro-organisms  and  micro- 
organisms. 

209.  Macro-organisms — animal  forms. — Of  the  macro- 
organisms  in  the  soil,  the  animal  types  are  chiefly  (1)  rodents, 
(2)  worms,  and  (3)  insects;  and  the  plant  forms  (1)  the 
large  fungi  and  algae,  and  (2)  roots. 

The  burrowing  habits  of  rodents — of  which  the  ground 
squirrel,  the  mole,  the  gopher,  and  the  prairie  dog  are  familiar 
examples — result  in  the  pulverization  of  considerable  quanti- 

1  General  references : 

Lipman,  J.  G.,   Bacteria  in  Eelation  to   Country   Life;   New   York, 
1908. 
Conn,  H.  W.,  Agricultural  Bacteriology;  Philadelphia,  1918. 
Marshall,  C.  E.,  Microbiology ;  Philadelphia,  1917. 

2  It  has  been  estimated  that  every  acre  of  soil  contains  at  least  2000 
pounds  of  living  material  exclusive  of  roots.  If  these  organisms  were 
confined  to  a  surface  foot  of  soil,  weighing,  when  moist,  4,000,000  pounds 
to  the  acre  foot,  they  would  make  up  .05  per  cent,  by  weight  of  the  nor- 
mal field  soil. 

384 


SOIL  ORGANISMS  385 

ties  of  soil.  While  the  effect  is  rather  beneficial  and  is  analo- 
gous to  tillage,  the  activities  of  these  animals  are  generally 
unfavorable  to  agricultural  operations  and  such  soil  inhabi- 
tants have  been  more  or  less  exterminated  in  arable  land. 

The  common  earthworm  is  the  most  conspicuous  example  of 
the  benefits  that  may  accrue  from  the  presence  of  animals. 
Darwin,  as  the  result  of  careful  measurements,  states  that  the 
quantity  of  soil  passed  through  these  creatures  may  under 
favorable  conditions  in  a  humid  climate,  amount  to  ten  tons 
of  dry  earth  to  the  acre  annually.  The  earthworm  obtains  its 
nourishment  from  the  organic  matter  of  the  soil,  but  takes 
into  its  alimentary  canal  the  inorganic  matter  as  well,  ex- 
pelling the  latter  in  the  form  of  casts  after  it  has  passed  en- 
tirely through  the  body.  The  ejected  material  is  to  some  ex- 
tent disintegrated,  and  is  in  a  flocculated  condition.  The  holes 
left  in  the  soil  serve  to  increase  aeration  and  drainage.  The 
activities  of  the  worms  bring  about  a  notable  transportation 
of  lower  soil  to  the  surface,  which  aids  still  more  in  effecting 
aeration.  Darwin's  studies  led  him  to  state  that  from  one- 
tenth  to  two-tenths  of  an  inch  of  soil  is  yearly  brought  to  the 
surface  of  land  in  which  earthworms  exist  in  numbers  normal 
to  fertile  soil. 

Earthworms  naturally  seek  a  heavy  compact  soil,  and  it  is 
in  soil  of  this  character  that  they  are  most  needed  because  of 
the  stirring  and  aeration  that  they  accomplish.  Sandy  soil 
and  that  of  arid  regions,  in  which  are  found  few  or  no  earth- 
worms, are  not  usually  in  need  of  their  activities. 

There  is  a  less  definite,  and  probably  a  less  effective,  action 
of  a  similar  kind  produced  by  insects.  Ants,  beetles,  and  the 
myriads  of  other  burrowing  insects  and  their  larvae  effect  a 
considerable  movement  of  soil  particles,  with  a  consequent 
aeration  of  the  soil.  At  the  same  time  they  incorporate  into 
the  soil  a  considerable  quantity  of  organic  matter. 

210.  Macro-organisms — plant  forms. — The  larger  fungi 
are  chiefly  concerned  in  bringing  about  the  first  stages  in  the 


386        NATURE  AND  PROPERTIES  OP  SOILS 

decomposition  of  woody  matter,  which  is  disintegrated  by  the 
penetrating  mycelia  of  the  fungi.  These  break  down  the 
structure,  and  thus  greatly  facilitate  the  work  of  the  decay 
bacteria.  Action  of  this  kind  is  largely  confined  to  the  forest 
and  is  not  of  great  importance  in  cultivated  soil.  Another 
function  of  the  large  fungi  is  exercised  in  the  intimate,  and 
possibly  symbiotic,  relation  of  the  fungal  hyphae  to  the  roots  of 
many  forest  trees,  in  soil  where  nitrification  proceeds  very 
slowly,  if  at  all,  for  nitrates  are  apparently  not  abundant  in 
forests. 

Algae,  except  in  special  cases,  do  not  exist  in  the  soil  to 
any  large  extent.  Certain  Colorado  soils,1  however,  seem  to 
contain  appreciable  numbers  of  this  form.  While  the  pres- 
ence of  both  the  larger  fungi  and  the  algse  is  interesting,  their 
importance  in  soil  fertility  is  probably  rather  slight. 

The  roots  of  plants  are  important  in  the  soil  both  by  con- 
tributing organic  matter  and  by  leaving,  on  their  decay,  open- 
ings which  render  the  soil  more  permeable  to  air  and  water. 
The  dense  mass  of  rootlets,  with  their  minute  hairs,  that  is  left 
in  the  soil  after  every  harvest,  furnishes  a  well-distributed 
supply  of  organic  matter,  which  is  not  confined  to  the  furrow 
slice,  as  is  artificially  incorporated  manure.  The  action  of 
roots  on  the  soil  is  not  by  any  means  entirely  physical.  Dur- 
ing the  life  of  the  plant  the  elimination  of  tissue  and  the 
presence  of  exudates  make  the  rootlets  rather  important  chem- 
ical agents.2  The  chemical  and  biological  importance  of  de- 
caying organic  matter  has  already  been  adequately  empha- 
sized.3 

211.  Micro-organisms — protozoa. — The  micro-organisms 
of  the  soil  belong  to  the  following  groups:  (1)  protozoa,  (2) 
fungi  and  algae,  (3)  actinomyces,  and  (4)  bacteria. 

1  Bobbins,  W.  W.,  Algce  in  Some  Colorado  Soils;  Colo.  Agr.  Exp.  Sta., 
Bui.  184,  1912. 

2  See  paragraphs  156  and  157. 

3  See  paragraphs     64  and  132. 


SOIL  ORGANISMS  387 

While  nematodes,  rotifers,  and  similar  organisms  are  some- 
times found  in  soil,  the  protozoa  are  the  only  important  micro- 
scopic animal  group  usually  present.  The  importance  of 
protozoa  in  soils  was  especially  emphasized  in  1909  by  Russell 
and  Hutchinson,1  who  maintained  that  the  protozoan  flora  so 
interfered  with  the  ammonia-producing  bacteria  as  materially 
to  lower  the  productivity  of  the  soil.  Partial  sterilization 
seemed  to  alleviate  this  condition,  possibly  by  killing  the 
harmful  protozoa.  The  findings  of  Russell  and  Hutchinson 
have  resulted  in  much  research  as  to  the  importance  of  proto- 
zoa in  a  normal  soil. 

While  Waksman  2  found  that  the  presence  of  protozoa  was 
concomitant  with  low  bacterial  numbers,  he  does  not  consider 
all  protozoa  harmful  to  biological  activities.  Fellers  and  Alli- 
son,3 in  an  examination  of  New  Jersey  soils,  found  protozoa  in 
every  sample,  the  number  of  species  ranging  from  two  to 
twenty-eight.  Soils  rich  in  organic  matter  or  containing  large 
amounts  of  water  carried  the  greater  number.  Besides  the 
104  species  of  protozoa  identified  in  New  Jersey  soils,  ten 
genera  of  alga?  and  six  of  diatomes  were  isolated.  Nematodes 
were  common.  The  number  of  protozoa  ranged  from  a  very 
few  to  as  high  as  4500  to  a  gram  of  soil.  When  occurring  in 
such  numbers,  these  animals  must  be  of  considerable  impor- 

1  Russell,  E.  G.,  and  Hutchinson,  H.  B.,  The  Effect  of  Partial  Sterili- 
zation of  Soil  on  the  Production  of  Plant  Food;  Jour.  Agri.  Sci.,  Vol. 
Ill,  pp.  111-144,  1909.  Also,  The  Effect  of  Partial  Sterilisation  of 
Soil  on  the  Production  of  Plant  Food.  II.  The  Limitation  of  Bac- 
terial Numbers  on  Soils  and  Its  Consequences;  Jour.  Agr.  Sci.,  Vol.  V, 
part  2,  pp.  152-221,  1913. 

a  Waksman,  S.  A.,  Protozoa  as  Affecting  Bacterial  Activities  in  the 
Soil;  Soil  Sci.,  Vol.  II,  No.  4,  pp.  363-376,  1916.  Also,  Sherman, 
J.  M.,  Studies  on  Soil  Protozoa  and  Their  Relation  to  the  Bacteria; 
I.  Jour.  Back,  Vol.  1,  No.  1,  pp.  35-66,  1916.  II.  Jour.  Back,  Vol.  1, 
No.  2,  pp.  165-184,  1916. 

Kopeloff,  N.,  and  Coleman,  D.  A.,  A  Review  of  Investigations  in 
Soil  Protozoa  and  Soil  Sterilization;  Soil  Sci.,  Vol.  Ill,  No.  3,  pp. 
197-269,  1917. 

"Fellers,  C.  R.,  and  Allison,  F.  E.,  The  Protozoan  Fauna  of  the 
Soil  of  New  Jersey;  Soil  Sci.,  Vol.  IX,  No.  1,  pp.  1-24,  1920. 


388        NATURE  AND  PROPERTIES  OF  SOILS 

tance  in  soils,  although  it  is  doubtful  whether  they  are  detri- 
mental except  under  special  conditions.1 

212.  Micro-organisms — fungi  and  algae. — Of  the  higher 
fungi,  molds  are  the  only  group  that  apparently  attain  any 
particular  importance  in  soils,  although  yeasts  have  been  found 
to  occur  and  may  in  special  cases  exist  in  considerable  num- 
bers. It  is  only  recently,  however,  that  fungi  have  received 
much  attention,  although  their  presence  has  been  noted  many 
times.  Such  common  genera  as  Fusarium,  Mucor,  Aspergillas, 
and  Pencillium  are  usually  present  in  normal  soils.  In  gen- 
eral, a  large  amount  of  organic  matter  is  conducive  to  the 
activity  of  such  fungi.  Molds  occur  in  soils  in  both  the  active 
and  the  spore  stage  and  probably  pass  their  various  life  cycles 
entirely  in  the  soil. 

Waksman,2  in  a  detailed  study  of  soil  fungi,  found  that 
most  of  the  organisms  were  capable  of  producing  considerable 
ammonia  from  nitrogenous  organic  matter.  A  large  propor- 
tion of  the  fungi  isolated  were  also  able  to  decompose  cellulose 
rather  rapidly.  Different  soils  seemed  to  have  a  distinct  and 
characteristic  fungal  flora.  Over  one  hundred  distinct  species 
of  fungi  were  isolated  by  Waksman  belonging  to  thirty-one 
genera.  Some  pathogenic  species,  such  as  different  Fusaria 
and  Alternaria,  were  found.  The  numbers  ranged  from  80,- 
000  to  a  gram  of  soil  under  forest  conditions  to  14,000,000 
to  a  gram  in  a  meadow  soil.    The  numbers  were  usually  larger 

1  Koch,  G.  P.,  Studies  on  the  Activity  of  Soil  Protozoa;  Soil  Sci., 
Vol.  II,  No.  2,  pp.  163-181,  1916. 

'Waksman,  S.  A.,  Soil  Fungi  and  Their  Activities;  Soil  Sci.,  Vol. 
II,  No.  2,  pp.  103-155,  1916.    Also, 

McLean,  H.  C,  and  Wilson,  G.  W.,  Ammonification  Studies  with  Soil 
Fungi;  N.  J.  Agr.  Exp.  Sta.,  Bui.  270,  1914. 

Kopeloff,  N.,  The  Effect  of  Soil  Reaction  on  Ammonification  by 
Certain  Soil  Fungi;  Soil  Sci.,  Vol.  V,  No.  1,  pp.  541-574,  1916. 

Coleman,  D.  A.,  Environmental  Factors  Influencing  the  Activity  of 
Soil  Fungi;  Soil  Sci.,  Vol.  V,  No.  2,  pp.  1-66,  1916. 

Brown,  P.  E.,  The  Importance  of  Mold  Action  in  Soil;  Science,  N.  S., 
Vol.  XLVI,  No.  1182,  pp.  171-175,  1917. 

Conn,  H.  J.,  The  Microscopic  Study  of  Bacteria  and  Fungi  in  Soil; 
N.  Y.  State  Agr.  Exp.  Sta.,  Tech.  Bui.  64,  1918. 


SOIL  ORGANISMS  389 

in  the  surface  soil.  While  the  microscopic  algae  are  probably- 
present  in  soils,  it  has  never  been  shown  that  they  are  of 
practical  importance. 

213.  Actinomyces. — The  actinomyces  are  a  filamentous 
form  of  organisms,  widely  distributed  in  nature  and  are  prob- 
ably more  nearly  related  to  the  bacteria  than  to  the  molds, 
although  they  produce  spores  and  develop  into  branching 
forms  of  considerable  complexity.  Their  production  of  aerial 
hyphae  is  quite  unlike  the  habits  of  bacteria.  These  thread 
organisms  exist  in  the  soil  in  both  the  vegetative  and  the 
resting  stage  and  often  make  up  quite  a  large  proportion  of 
the  soil  flora.  They  are  extremely  difficult  to  study,  since 
they  produce  hard  compact  growths.  It  is  questionable  also, 
whether  the  growths  produced  artificially  are  exactly  like 
those  occurring  in  the  soil. 

Hiltner  and  Stormer  *  found  that  20  per  cent,  of  the  soil 
organisms  developing  on  gelatin  plates  inoculated  from  the 
soil  were  actinomyces.  Conn  2  reports  a  range  from  11  to  75 
per  cent,  under  similar  cultural  conditions.  The  average  was 
38  per  cent.  Conn  estimates  that  20  per  cent,  of  the  average 
flora  consists  of  actinomyces.  The  organisms  were  generally 
greater  in  meadow  soil  than  in  cultivated  land,  indicating 
the  relationship  of  these  thread  forms  to  cellulose  decomposi- 
tion. McBeth 3  found  actinomyces  of  wide  distribution  in 
soils  and  he  concludes  that  they  are  undoubtedly  an  impor- 
tant factor  in  the  decomposition  of  the  cellulose  of  the  soil 
organic  matter. 

1  Hiltner,  L.,  and  Stormer,  K.,  Studien  uber  die  Bdkterienflora  des 
Ackerbodens  ;  Kaiserliches  Gesundheitsamt,  Biol.  Abt.  Land-u.  Forstw., 
Bd.  3,  S.  445-545,  1903. 

2  Conn,  H.  J.,  A  Possible  Function  of  Actinomycetes  in  Soil;  Jour. 
Bact.,  Vol.  1,  No.  2,  pp.  197-207,  1916. 

3  McBeth,  I.  G.,  Studies  on  the  Decomposition  of  Cellulose  in  Soils; 
Soil  Sci.,  Vol.  I,  No.  5,  pp.  437-487,  1916.     Also, 

Waksman,  S.  A.,  and  Curtis,  R.  E.,  The  Actinomyces  of  the  Soil; 
Soil  Sci.,  Vol.  1,  No.  2,  pp.  99-134,  1916. 

Waksman,  S.  A.,  Cultural  Studies  of  Species  of  Actinomyces;  Soil 
Sci.,  Vol.  VIII,  No.  2,  pp.  71-207,  1919. 


390         NATURE  AND  PROPERTIES  OF  SOIL 

214.  Bacteria. — Of  the  several  forms  of  micro-organisms 
in  the  soil,  bacteria  are  probably  the  most  important.  In  fact, 
the  abundant  and  continued  growth  of  higher  plants  on  the 
soil  is  absolutely  dependent  on  the  presence  of  bacteria. 
Through  their  action  chemical  changes  are  brought  about 
which  result  in  the  solution  of  both  organic  and  inorganic 
material  necessary  for  the  life  of  higher  plants,  and  which, 
in  part  at  least,  would  not  otherwise  be  available. 

Bacteria  are  single  cell  organisms  and  are  probably  the 
simplest  forms  of  life  with  which  we  have  to  deal.  They  are 
generally  much  smaller  than  yeasts,  multiplying  by  elongat- 
ing and  dividing  into  half.  They  are,  therefore,  often  called 
fission  fungi.  Molds  multiply  by  budding.  The  activities  of 
both  groups  are  similar,  in  that  they  produce  their  effects 
very  largely  by  the  production  of  enzymes.1  The  importance 
of  enzymic  influences  must  constantly  be  borne  in  mind  in  all 
biological  transformations  in  the  soil. 

Bacteria  are  very  small,  the  larger  individuals  seldom  ex- 
ceeding one  or  two  microns  (.001  to  .002  m.m.)  in  diameter. 
In  the  soil  there  is  good  reason  to  suppose  that  there  are 
many  groups  which  are  too  small  to  be  seen  under  the  micro- 
scope. Such  organisms  may,  therefore,  function  as  a  part  of 
the  colloidal  matter  of  the  soil.  Many  of  the  soil  bacteria 
are  equipped  with  extremely  delicate  vibrating  hairs  called 
flagella,  which  enable  the  organisms  to  swim  through  the 

1  Bacteria,  as  well  as  most  fungi,  bring  about  their  important  trans- 
formations largely  by  means  of  enzymes.  These  enzymes  are  catalytic 
agents  and  are  generally  considered  as  colloidal  in  nature.  A  number  of 
transformations  may  be  accelerated  by  enzymes,  the  exact  reaction  de- 
pending on  the  nature  of  the  enzyme  itself.  The  change  in  the  soil  of 
ammonia  (NH,)  to  the  nitrate  form  (NOs)  is  an  example  of  oxidation 
and  is  spoken  of  as  nitrification.  The  reversal  of  this  action  is  desig- 
nated as  reduction  and  is  probably  not  entirely  enzymic.  A  splitting 
action  is  very  common.  The  breaking  up  of  glucose  into  alcohol  and 
carbon  dioxide  is  an  example  of  this  ((VS^O,  =  2C2H5OH  +  2COa). 
A  fourth  reaction  that  may  be  hastened  by  enzymic  influence  is  hydrol- 
ysis. Cane-sugar  may  thus  quickly  produce  glucose  and  fructose 
(C^H^On  -f  H20  =  CeH^Oe  +  C.H^O,). 


SOIL  ORGANISMS  391 

soil-water.  The  shape  of  bacteria  is  varied  in  that  they  may 
be  nearly  round,  rod-like,  or  spirals.  In  the  soil  the  rod- 
shaped  organisms  seem  to  predominate. 

As  already  stated,  the  primary  method  of  multiplication 
of  bacteria  is  by  simple  division,  the  process  being  very  rapid 
under  favorable  conditions.  The  phenomena  frequently  takes 
place  in  thirty  minutes.  This  almost  unlimited  capacity  to 
increase  in  numbers  is  extremely  important  in  the  soil  since 
it  allows  certain  groups  quickly  to  assume  their  normal  func- 
tions under  favorable  conditions,  even  though  their  numbers 
were  originally  small.1  Bacteria  may  thus  be  considered  as 
a  force  of  tremendous  magnitude  in  the  soil,  held  more  or 
less  in  check  by  conditions,  but  ever  ready  to  exert  an  influ- 
ence of  profound  importance  on  crop  growth. 

In  the  soil  bacteria  probably  exist  as  mats  or  clumps, 
called  colonies,  on  and  around  the  soil  particles  wherever  food 
conditions  are  favorable.  Natural  and  artificial  forces  tend 
to  break  up  these  colonies  and,  as  many  groups  are  flagellated, 
bacteria  becomes  well  distributed  through  the  soil.  In  gen- 
eral the  greatest  numbers  are  found  in  the  surface  layers  of 
the  soil,  since  conditions  of  temperature,  aeration,  and  food 
are  here  more  favorable.  Many  of  the  soil  bacteria  are  able 
to  produce  spores,  thus  presenting  both  a  resting  and  a  vege- 
tative stage.  The  production  of  spores  is  often  extremely 
important  as  it  allows  the  organisms  to  survive  unfavorable 
conditions  of  many  kinds. 

The  number  of  bacteria  present  in  soil  is  quite  variable  as 
many  conditions  markedly  affect  their  growth.  The  meth- 
ods 2  of  determining  the  numbers  are  extremely  inaccurate, 

1  If  a  single  bacterium  and  every  subsequent  organism  produced  sub- 
divided every  hour,  the  offspring  from  the  original  cell  would  be  about 
17,000,000  in  twenty-four  hours.  In  six  days  the  organisms  would  greatly 
surpass  the  earth  in  volume.  Under  actual  conditions  such  multiplication 
would  never  occur,  due  to  lack  of  food  and  other  limitations. 

2  The  counting  of  soil  bacteria  is  generally  carried  out  somewhat  as 
follows:  A  small  sample  of  soil  (usually  .5  gram)  is  placed  in  a  sterile 
Erlenmeyer  flask  and  treated  with  100  cc.  of  sterile  water.     The  sample 


392        NATURE  AND  PROPERTIES  OF  SOILS 

since  many  organisms  cannot  grow  in  the  artificial  media 
commonly  used.  Moreover,  it  is  almost  impossible  to  break 
up  the  clump  of  colonies  in  such  a  way  as  to  determine  the 
number  of  individuals  present.  It  is  fairly  certain,  however, 
that  the  numbers  of  bacteria  in  soil  are  very  large,  possibly 
ranging  from  500,000  to  100,000,000  to  a  gram  of  dry  soil. 
Good  soils  seem,  in  general,  to  carry  the  greatest  numbers. 
The  bacterial  flora,  as  well  as  the  other  soil  organisms,  fluctu- 
ate markedly  with  season,  the  numbers  usually  being  great- 
est in  the  summer  months. 

215.  Conditions  affecting  bacterial  growth.1 — Many  con- 
is  then  well  shaken  in  order  to  produce  a  suspension  containing  the 
bacteria  originally  present  in  the  soil.  Dilutions  of  1  to  20,000,  1  to 
100,000  and  1  to  200,000  based  on  the  original  soil  sample  are  made. 
Gelatin  or  agar  plates  are  then  inoculated,  three  from  each  dilution. 
After  adequate  incubation  the  colonies  on  the  plates  are  counted,  each 
colony  supposedly  representing  one  original  organism.  The  numbers  of 
bacteria  that  were  present  in  the  original  soil  are  then  calculated.  The 
agar  or  gelatin  of  the  plates  generally  receive  a  sterile  extract  from  the 
soil  together  with  certain  added  materials,  organic  or  inorganic,  in  order 
that  the  growth  of  the  bacteria  may  be  hastened. 

Such  a  count  does  not  represent  by  any  means  all  of  the  bacteria  of 
the  soil,  as  some  groups  will  not  develop  at  all,  while  others  require 
special  media.  Slowly  growing  groups  of  organisms,  that  would  prob- 
ably appear  if  time  were  given,  escape  the  count,  since  the  plates  are  so 
quickly  covered  by  more  abundant  growths.  The  suspension  from  the 
soil,  used  to  inoculate  the  plates,  does  not  contain  all  of  the  organisms 
as  single  individuals,  since  it  is  impossible  completely  to  break  down  the 
clump  formation.  This  tends  to  make  the  counts  too  low.  Special 
media  and  technique  are  of  course  necessary  in  studying  fungi,  algse 
and  actinomyces. 

1  Rahn,  Otto,  The  Bacterial  Activity  in  Soil  as  a  Function  of  Grain-size 
and  Moisture  Content;  Mich.  Agr.  Exp.  Sta.,  Tech.  Bui.  16,  1912. 

Plummer,  J.  K.,  Some  Effects  of  Oxygen  and  Carbon  Dioxide  on  Nitri- 
fication and  Ammonification  in  Soils;  Cornell  Agr.  Exjy  Sta.,  Bui.  384, 
1916. 

Greaves,  J.  E.,  and  Carter,  E.  G.,  Influence  of  Barnyard  Manure 
and  Water  Upon  the  Bacterial  Activities  of  the  Soil;  Jour.  Agr.  Res., 
Vol.  VI,  No.  23,  pp.  889-926,  1916. 

Brown,  P.  E.  The  Influence  of  Some  Common  Humus-forming  Mate- 
rials of  Narrow  and  of  Wide  Nitrogen-carbon  Ratio  on  Bacterial  Num- 
bers; Soil  Sci.,  Vol.  1,  No.  1,  pp.  49-75,  1916. 

Waksman,  S.  A.,  Bacterial  Numbers  in  Soils,  at  Different  Depths  and 
in  Different  Seasons  of  the  Year;  Soil  Sci.,  Vol.  I,  No.  4,  pp.  363-380, 
1916. 

Gainey,  P.  L.,  The  Effect  of  Time  and  Depth  of  Cultivating  a  Wheat 


SOIL  ORGANISMS  393 

ditions  of  the  soil  affect  the  growth  of  bacteria.  Among  the 
most  important  of  these  are  the  supply  of  oxygen  and  mois- 
ture, the  temperature,  the  presence  of  organic  matter,  and 
the  acidity  or  the  basicity  of  the  soil. 

All  soil  bacteria  require  for  their  growth  a  certain  amount 
of  oxygen.  Some  bacteria,  however,  can  continue  their  activ- 
ities with  much  less  oxygen  than  can  others.  Those  requir- 
ing an  abundant  supply  of  oxygen  have  been  called  aerobic 
bacteria,  while  those  preferring  little  air  are  designated  as 
anaerobic  bacteria.  This  is  an  important  distinction,  because 
those  bacteria  that  are  of  greatest  benefit  to  the  soil  are,  in 
the  main  aerobes,  and  those  that  are  injurious  in  their  action 
are  chiefly  anaerobes.  However,  it  seems  likely  that  an 
aerobic  bacterium  may  gradually  accommodate  itself  within 
certain  limits  to  an  environment  containing  less  oxygen,  and 
an  anaerobic  bacterium  may  accommodate  itself  to  the  pres- 
ence of  a  larger  amount  of  oxygen.  It  is  quite  possible  that 
the  aerobic  and  anaerobic  organisms  function  in  the  soil  at 
the  same  time,  since  a  portion  even  of  a  well  aerated  soil  is 
always  highly  charged  with  carbon  dioxide.  It  is  not  improb- 
able, also,  that  there  exists  a  more  or  less  beneficial  inter- 
relation between  the  two  general  groups. 

Bacteria  require  moisture  for  their  growth,  optimum  water 
for  higher  plants  seemingly  being  the  best  moisture  for  the 
development  and  activity  of  favorable  soil  organisms  of  all 
kinds.  With  a  decrease  of  moisture  the  soil  becomes  well 
aerated,  while  an  excessive  water  supply  tends  to  encourage 
anaerobic  conditions.  Moisture,  when  aeration  and  tempera- 
ture are  favorable,  seems  to  be  the  main  control  of  biological 
changes  within  the  soil. 

Soil  bacteria,  like  other  plants,  continue  life  and  growth 

Seed-Bea  upon  Bacterial  Activity  in  the  Soil;  Soil  Sei.,  Vol.  II,  No.  2, 
pp.  193-204,  1916. 

Greaves,  J.  E.,  and  Carter,  E.  G.,  Influence  of  Moisture  on  the  Bac- 
terial Activities  of  the  Soil;  Soil  Sci.,  Vol.  X,  No.  5,  pp.  361-387,  1920. 


394        NATURE  AND  PROPERTIES  OF  SOILS 

under  a  considerable  range  of  temperature.  Freezing,  while 
rendering  bacteria  dormant,  does  not  kill  them,  and  growth 
begins  slightly  above  that  point.1  It  has  been  shown  that 
some  nitrification  occurs  at  temperatures  as  low  as  from  37° 
to  39°  F.  It  is  not,  however,  until  the  temperature  is  con- 
siderably higher  that  bacterial  functions  are  pronounced. 
From  70°  to  110°  F.  their  activity  is  greatest,  and  it  dimin- 
ishes perceptibly  below  or  above  those  points.  The  thermal 
death  point  of  most  forms  of  bacteria  is  between  110°  and 


Fig.  56. — Some  important  decay  organisms  found  in  soils,  (a),  Acti- 
nomyces threads;  (b),  a  colony  of  Actinomyces;  (e)  and  (d),  Pro- 
teus vulgaris;  (e),  B.  fluorescens;  (f ),  B.  subtilis. 

160°  F.,  but  the  spore  forms  even  resist  boiling.  Only  in 
some  desert  soils  does  the  natural  temperature  reach  a  point 
sufficiently  high  actually  to  destroy  bacteria,  and  there  only 
near  the  surface.  In  fact,  it  is  very  seldom  that  soil  tempera- 
tures, other  conditions  being  favorable,  become  sufficiently 
high  to  curtail  bacterial  activity. 

The  presence  of  a  certain  amount  of  organic  matter  is  es- 
sential to  the  growth  of  most,  but  not  all,  forms  of  soil  bac- 

1  In  the  seasonal  study  of  bacteria  it  has  been  repeatedly  noticed  that 
the  counts  increased  during  the  winter,  especially  after  a  freeze  followed 
by  a  thaw.  It  was  considered  for  a  time  that  a  special  winter  flora  was 
present,  and  was  able  to  multiply  in  the  soil-water  which  failed  to  freeze. 
It  is  now  considered  that  this  increase  is  only  apparent,  the  freezing 
having  disrupted  the  bacterial  clumps,  thus  increasing  the  number  of 
colonies  appearing  on  the  plates  during  incubation. 


SOIL  ORGANISMS  395 

teria.  The  organic  matter  of  the  soil,  consisting  as  it  does  of 
the  remains  of  a  large  variety  of  substances,  furnishes  a  suit- 
able food  supply  for  a  very  great  number  of  forms  of  organ- 
isms. The  action  of  one  set  of  bacteria  on  the  cellular  matter 
of  plants  embodied  in  the  soil  produces  compounds  suited  to 
other  forms,  and  so  from  one  stage  of  decomposition  to  another 
this  constantly  changing  material  affords  sustenance  to  bac- 
terial flora,  the  extent  and  variety  of  which  it  is  difficult  to 
conceive.  A  soil  low  in  organic  matter  usually  has  a  lower 
bacterial  content  than  one  containing  a  large  amount,  and, 
under  favorable  conditions,  the  beneficial  action,  to  a  certain 
point  at  least,  increases  with  the  content  of  organic  substance ; 
but,  as  the  products  of  bacterial  life  are  generally  injurious 
to  the  organisms  producing  them,  such  factors  as  the  rate 
of  aeration  and  the  basicity  of  the  soil  must  determine  the 
effectiveness  of  the  organic  matter. 

The  so-called  acidity  of  the  soil  is  probably  as  important 
a  factor  in  bacterial  activity  as  it  is  to  higher  plants..  In 
general,  favorable  soil  organisms  of  all  kinds  seem  to  func- 
tion better  in  a  soil  carrying  sufficient  active  base  to  generate 
conditions  favorable  for  higher  plants.  An  exception  some- 
times occurs,  however,  notably  in  the  case  of  the  "finger-and- 
toe"  disease  of  certain  Crucifera3,  which  is  retarded  by 
liming. 

The  activities  of  many  soil  bacteria  result  in  the  formation 
of  acids  which  are  injurious  to  the  bacteria  themselves,  and 
unless  there  is  present  some  base  with  which  these  can  com- 
bine, bacterial  development  is  inhibited  by  such  products. 
This  is  one  of  the  reasons  why  lime  is  so  often  of  great  benefit 
when  applied  to  soils,  and  especially  to  those  on  which  alfalfa 
and  red  clover  are  growing.  For  the  same  reason  the 
presence  of  lime  hastens  the  decay  of  organic  matter  in 
certain  soils,  and  the  conversion  of  nitrogenous  material 
into  compounds  available  to  the  plants.  As  showing  the 
value  of  lime  in  the  process  of  nitrate  formation  it  has  been 


396         NATURE  AND  PROPERTIES  OF  SOILS 

pointed  out  that  in  the  presence  of  an  adequate  supply  of 
calcium  the  availability  of  ammonium  salts  is  almost  as  high 
as  that  of  nitrate  salts,  but  where  the  supply  of  calcium  is 
insufficient  the  value  of  ammonium  salts  is  relatively  low. 

216.  Organisms  injurious  to  higher  plants. — While  the 
macro-organisms  may,  under  certain  conditions,  be  detri- 
mental to  the  growth  of  higher  plants,  it  is  the  smaller  in- 
habitants of  the  soil  that  attract  especial  attention  in  this  re- 
spect. While  protozoa  may,  under  special  circumstances,  be 
extremely  detrimental,  injurious  organisms  are  confined 
mostly  to  fungi  and  bacteria.  They  may  be  entirelv  parasitic 
in  their  habits  or  only  partially  so,  while  they  may  injure 
higher  plants  by  attacking  the  roots  or  even  the  tops.  Those 
that  infest  parts  of  the  plant  other  than  the  roots  are  not 
strictly  soil  organisms,  as  they  pass  only  a  part  of  their 
cycle  in  the  soil.  Some  of  the  more  common  diseases  pro- 
duced by  soil  organisms  are :  wilt  of  cotton,  cowpeas,  water- 
melon, flax,  tobacco,  tomatoes,  and  other  plants;  damping-off 
of  a  large  number  of  plants ;  root-rot ;  and  galls. 

Injurious  fungi  or  bacteria  may  live  for  long  periods  in 
the  soil,  if  the  conditions  necessary  for  their  growth  are  main- 
tained. Some  of  them  will  die  within  a  few  years  if  their  host 
plants  are  not  grown  on  the  soil,  but  others  are  able 'to  main- 
tain existence  on  almost  any  organic  substance.  Once  a 
soil  is  infected  it  is  likely  to  remain  so  for  a  long  time,  or 
indeed  indefinitely.  Infection  easily  occurs.  Organisms  from 
infected  fields  may  be  carried  on  implements,  plants,  or  rub- 
bish of  any  kind,  in  soil  used  for  inoculation  of  leguminous 
crops,  or  even  in  stable  manure  containing  infected  plants 
or  in  the  feces  resulting  from  the  feeding  of  such  plants. 
Flooding  of  land  by  which  soil  is  washed  from  one  field  to 
another  may  be  a  means  of  infection. 

Prevention  is  the  best  defense  from  diseases  produced  by 
such  soil  organisms.  Once  a  disease  has  procured  a  foothold, 
it  is  often  impossible  to  eradicate  all  its  organisms.     Rota- 


SOIL  ORGANISMS  397 

tion  of  orops  is  effective  for  some  diseases,  but  entire  absence 
of  the  host  crop  is  often  necessary.  The  use  of  lime  is  bene- 
ficial in  the  case  of  certain  diseases.  Chemicals  of  various 
kinds  have  been  tried  with  little  success.  Steam  sterilization 
is  a  practical  method  of  treating  greenhouse  soils  for  a  num- 
ber of  diseases.  The  breeding  of  plants  immune  to  the  dis- 
ease affecting  its  particular  species  has  been  successfully  car- 
ried out  in  the  case  of  the  cowpea  and  cotton,  and  can  doubt- 
less be  accomplished  with  others. 

In  regions  in  which  farming  is  confined  largely  to  one 
crop  or  to  a  limited  number  of  cereals,  it  is  the  common  ex- 
perience that  yields  decrease  greatly  in  the  course  of  a  score 
of  years  after  the  virgin  soil  is  broken.  The  cause  for  this 
is  attributed  by  Bolley,1  in  large  measure,  to  a  diseased  con- 
dition of  the  plants,  due  to  the  growth  of  various  fungi  that 
inhabit  the  soil  and  attack  the  crops  grown  on  it.  He  reports 
that  he  experimented  with  pure  cultures  taken  from  wheat 
grains,  straw,  and  roots,  and  has  demonstrated  that  certain 
strains  or  species  of  Fusarium,  Helminthosporium,  Alter- 
naria,  Macrosporium,  Colletotrichum,  and  Cephalothecium 
are  directly  capable  of  attacking  and  destroying  growing 
plants  of  wheat,  oats,  barley,  brome-grass,  and  quack-grass, 
and  that  within  limits  the  disease  may  be  transferred  from 
one  type  of  crop  to  another. 

217.  The  beneficial  influences  of  soil  organisms. — While 
the  macro-organisms  of  the  soil  are  usually  beneficial  to 
higher  plants,  the  more  important  relationships  are  occupied 
by  the  micro-organisms.  The  micro-organisms  of  the  soil  take 
an  active  part  in  removing  dead  plants  and  animals  from  the 
surface  of  the  land,  and  in  bringing  about  the  other  oper- 
ations that  are  necessary  for  the  production  of  higher  plants. 
The  first  step  in  preparation  for  plant  growth  is  to  remove  the 
remains  of  plants  and  animals  that  would  otherwise  accumu- 
late to  the  exclusion  of  higher  plants.  These  are  decomposed 
1  Bolley,  H.  L.,  Wheat;  N.  Dak.  Agr.  Exp.  Sta.,  Bui.  107,  1913. 


398         NATURE  AND  PROPERTIES  OF  SOILS 

through  the  action  of  organisms  of  various  kinds,  the  inter- 
mediate and  final  products  of  decomposition  assisting  plant 
production  by  contributing  nitrogen,  and  certain  mineral 
compounds  that  are  a  directly  available  source  of  plant  nutri- 
ents, and  also  by  the  effect  of  certain  of  the  decomposition 
products  on  the  mineral  substances  of  the  soil,  by  which  they 
are  rendered  soluble  and  hence  available  to  plants. 

Through  these  operations  the  supply  of  carbon  and  nitro- 
gen required  for  the  production  of  organic  matter  is  kept  in 
circulation.  The  complex  organic  compounds  in  the  bodies 
of  dead  plants  or  animals,  in  which  condition  higher  plants 
cannot  use  them,  are,  under  the  action  of  micro-organisms, 
converted  by  a  number  of  stages  into  the  simple  compounds 
used  by  plants.  In  the  course  of  this  process,  a  part  of  the 
nitrogen  is  sometimes  lost  into  the  air  by  conversion  into  free 
nitrogen,  but  fortunately  this  may  be  recovered  and  even 
more  nitrogen  taken  from  the  air  by  certain  other  organisms 
of  the  soil. 

Higher  fungi  and  actinomyces  are  particularly  active  in 
the  early  stages  of  decomposition  of  both  nitrogenous  and 
non-nitrogenous  organic  matter.  Molds  are  capable  of  am- 
monifying proteins,  and  even  re-forming  complex  protein 
bodies  from  the  nitrogen  of  ammonium  salts.  Certain  of  the 
molds  and  of  the  algae  are  apparently  able  to  fix  atmospheric 
nitrogen,  and  contribute  in  addition  a  supply  of  carbohy- 
drates required  for  the  use  of  the  nitrogen-fixing  bacteria. 
While  the  higher  fungi  are  important  in  such  transforma- 
tions, their  activities  in  almost  every  stage  are  excelled  by 
those  of  the  bacteria.  Because  of  this,  the  vital  biological 
transformations  within  the  soil  are  generally  ascribed  to  bac- 
terial action,  the  bacteria  receiving  the  greatest  attention  of 
the  numberless  organisms  making  up  both  the  soil  flora  and 
fauna. 

218.  Biological  cycles.— Because  of  a  lack  of  knowledge 
regarding  the  flora  and  fauna  of  the  soil,  it  is  obviously  im- 


SOIL  ORGANISMS  399 

possible  to  discuss  in  detail  the  transformations  caused  by 
individual  species  of  organisms  or  even  by  groups  of  related 
species.  From  the  standpoint  of  soil  fertility  such  an  at- 
tempt is  unnecessary,  as  a  practical  understanding  of  the 
changes  through  which  a  given  soil  constituent  passes  as 
it  is  prepared  for  plant  nutrition,  is  much  more  important 
than  the  possession  of  specific  knowledge  regarding  the  organ- 
isms concerned.  As  a  consequence  it  has  become  customary 
to  discuss  the  biological  transformations  of  the  more  impor- 
tant soil  constituents,  including  as  much  regarding  the  speci- 
fic organisms  and  groups  of  organisms  involved  as  is  con- 
sistent with  a  clear  fertility  viewpoint.1  Four  cycles  are  gen- 
erally recognized,  as  follows:  (1)  the  carbon  cycle,  (2)  the 
sulfur  cycle,  (3)  the  mineral  cycle,  and  (4)  the  nitrogen 
cycle. 

219.  The  carbon  cycle. — Since  all  organic  compounds 
carry  carbon,  nitrogenous  as  well  non-nitrogenous  materials 
are  involved  in  the  carbon  cycle.  Nevertheless  attention  will 
be  directed  for  the  time  being  only  toward  the  carbon  and  the 
changes  that  it  undergoes  from  the  time  it  enters  the  soil 
until  it  is  removed  either  by  aeration,  leaching,  or  by  plant 
absorption. 

Most  of  the  carbon  compounds  enter  the  soil  as  plant  tissue, 
although  animal  remains  contribute  appreciable  amounts. 
These  carbonaceous  materials  are  immediately  attacked  in  the 
soil  by  a  host  of  different  organisms  capable  of  producing 
fermentation.  While  such  bacteria  as  Bacillus  subtilis,  Ba- 
cillus mycoides,  and  the  like  have  a  great  deal  to  do  with  the 
decay  processes,  they  are  by  no  means  the  only  agents.  Most 
of  the  microscopic  fungi,  as  well  as  the  larger  fungi  and  algae, 

1  There  are  two  general  ways  of  studying  the  soil  flora.  A  classification 
of  the  organisms  may  be  attempted.  This  requires  the  isolation  and 
study  of  individuals  and  has  so  far  met  with  but  little  success.  The 
second  approach  is  a  biochemical  one,  in  which  the  transformations  oc- 
curring in  the  soil  are  studied  first,  the  specific  organisms  involved  being 
a  secondary  consideration.  The  determination  of  the  capacity  of  the 
soil  to  produce  ammonia  is  an  example  of  this  method  of  study. 


400         NATURE  AND  PROPERTIES  OF  SOILS 

aid  in  the  initial  transformation,  being  particularly  effective 
in  decomposing  cellulose.  The  actinomyces,  present  in  such 
large  numbers,  seem  to  be  especially  fitted  for  the  breaking 
down  of  such  resistant  material. 

The  result  of  these  complex  decomposition  processes  is  the 
formation  of  a  partially  decayed  group  of  carbon-bearing 
material,  some  being  quite  simple  while  others  are  extremely 
complicated.  The  change  is  accompanied  through  its  entire 
course  by  the  formation  of  carbon  dioxide  and  water,  the  end- 
products  of  carbohydrate  decay.  The  same  heterogeneous 
group  of  soil  organisms,  which  initiate  the  simplification  of 
carbonaceous  materials,  seem  to  continue  the  process  until 
only  the  end  products  and  the  more  resistant  portions  of  the 
original  tissue  remain. 

The  transformations  above  discussed  are  not  the  only 
sources  of  carbon  dioxide  within  the  soil.  Some  carbon  diox- 
ide is  brought  down  in  rain-water,  while  still  more  is  given  off 
by  the  roots  of  living  plants  (see  par.  156).  Moreover  some 
carbon  dioxide  is  obtained  from  the  inorganic  matter  of  the 
soil,  especially  if  the  land  has  recently  received  an  applica- 
tion of  limestone.  The  reactions  within  the  soil  seem  to  de- 
compose such  carbonates  rather  readily,  carbon  dioxide  being 
given  off  (see  par.  201). 

220.  The  loss  of  carbon  from  the  soil. — Carbon  diox- 
ide, the  importance  of  which  has  already  been  fully  discussed 
(par.  132)-  may  suffer  transformation  in  a  number  of  ways 
in  the  soil.  It  may  be  lost  (1)  to  the  atmospheric  air;  (2) 
it  may  react  with  the  mineral  constituents  of  the  soil  and  be 
held  at  least  temporarily  by  the  soil  mass;  or  (3)  it  may  be 
removed  by  leaching.  Since  the  soil-water  is  always  more  or 
less  charged  with  carbon  dioxide  and  since  'it  carries  car- 
bonate and  bicarbonate  salts,  considerable  carbon  is  continu- 
ally being  removed  in  this  way.  In  this  regard  the  figures 
from  the  Cornel  lysimeter  tanks1  are  especially  interesting. 

1  Unpublished  data.    Cornell  Agr.  Exp.  Sta.,  Ithaca,  N.  Y. 


SOIL  ORGANISMS 


401 


The  data  are  expressed  in  pounds  to  the  acre  and  are  averages 
of  ten  years'  experimentation.  The  carbon  was  lost  as  the 
bicarbonate,  only  traces  of  carbonates  being  present.  (See 
table  LXXXVIII,  page  402). 

I  ft    ^02 

ANIMAL: 


GREEN  FARM 

MANURE  MANURE 


v  V 


-  DECAY 
PARTIALLY  DECOMPOSED 
MATERIALS 


OTHER    BIOLOGICAL 
ACTIVITIES 


LEACHING 
LOSSES 


CHEMICAL 
REACTIONS 


Fig.   57. — Diagram  showing  the   transformations   of   carbon,   commonly 
spoken  of  as  the  "carbon  cycle. " 


It  is  apparent  that  a  drainage  loss  of  about  1200  pounds 
of  bicarbonate  (HC03)  may  be  expected  each  year  to  the 
acre,  without  considering  the  carbon  dioxide  which  is  respired 
to  the  atmosphere.  This  latter  loss  probably  at  least  equals, 
if  it  does  not  greatly  exceed,  the  loss  of  carbon  in  the  bicar- 
bonate form.  Together  they  cause  a  disappearance  of  several 
hundred  pounds  of  carbon  a  year  under  the  conditions  main- 


402        NATURE  AND  PROPERTIES  OF  SOILS 
Table  LXXXVIII 

LOSS   OF    CARBON    FROM    THE    SOIL    IN    DRAINAGE,    EXPRESSED    IN 
POUNDS   TO  THE  ACRE  PER  YEAR.      CORNELL   LYSI METERS. 


Treatment 


Carbon 
(pounds) 


Bare   soil 1391  273 

Rotation 1350  265 

Grass 1193  234 

tained  in  the  Cornell  lysimeters.  The  application  of  two  tons 
of  green-manure  to  the  acre  would  be  necessary  to  replace 
even  the  drainage  loss  cited  above. 

Small  amounts  of  carbon  may  be  removed  by  means  other 
than  drainage  or  diffusion  into  the  atmospheric  air.  Nu- 
merous investigators x  have  shown  that  plants  are  capable 
of  assimilating  various  organic  materials.  Recently  it  has 
been  demonstrated  that  higher  plants  may  utilize  a  consid- 
erable variety  of  carbohydrate  compounds.2  Such  materials, 
when  thus  assimilated,  no  doubt  supply  the  plant  with  en- 
ergy and  thus  are  foods  rather  than  nutrients.  The  ready 
response  of  certain  crops,  such  as  maize,  to  applications  of 
farm  manure  lends  plausibility  to  the  theory  that  considerable 
carbon  may  be  removed  from  the  soil  by  plants  and  that  the 
carbon  dioxide  of  the  air  is  not  the  only  immediate  source  of 
the  element  carbon. 

221.     The  sulfur  cycle.— Sulfur  is  an  essential  plant  nu- 

1  Hutchinson,  H.  B.,  and  Miller,  N.  H.  J.,  The  Direct  Assimilation  of 
Inorganic  and  Organic  Forms  of  Nitrogen  by  Higher  Plants:  Centrlb.  f. 
Bakt.,  II,  Band  30,  S.  513-547,  1911. 

2  Maze,  P.,  Influence,  sur  le  developpement  de  la  plante,  des  substances 
minerales  qui  s'accumulent  dans  ses  organes  comme  residus  d' assimila- 
tion; Compt.  Rend.  Sci.,  Paris,  Tome  152,  pp.  783-785,  1911. 

Ravin,  P.,  Nutrition  carbonee  des  plantes  a  Vaide  des  acides  organique 
libres  et  combines;  Ann.  Sci.  Nat.  Bot.,  Ser.  9,  No.  18,  pp.  289-446, 
1913. 

Knudson,  L.,  Influence  of  Certain  Carbohydrates  on  Green  Plants; 
Cornell  Agr.  Exp.  Sta.,  Memoir  9,  July  1916. 


SOIL  ORGANISMS  403 

trient,  being  utilized  by  such  crops  as  alfalfa,  turnips,  and 
cabbage  in  much  larger  amounts  than  is  phosphorus.  Al- 
though sulfur  probably  seldom  becomes  a  limiting  factor  in 
crop  production  (see  par.  264),  where  rational  methods  of 
soil  management  are  practiced,  its  transformations  in  the  soil 
are  of  great  importance. 

Sulfur  is  absorbed  by  the  plant  as  the  sulfate  ion  and  con- 
sequently all  forms  of  soil  sulfur  must  be  changed  to  the  sul- 
fate before  the  plant  may  benefit  to  any  degree.  This  trans- 
formation of  sulfur,  both  organic  and  inorganic,  to  the  sul- 
fate form,  insofar  as  it  is  biological,  has  been  termed  by 
Lipman,1  sulfofication.  The  reactions  involved  after  hydro- 
gen sulfide  or  free  sulfur  are  formed  may  be  written  as  fol- 
lows: 

2H2S  +  302  as  2H2S03 

S  +  H20  +  02  =  H2S03 

H2S03  +  CaH2(C03)2  =  CaS03  +  2H20  +  2C02 

2CaS03  +  02  =  2CaS04 

While  the  oxidation  reactions  cited  above  are  not  entirely 
biological,  purely  chemical  changes  occurring  to  a  slight  de- 
gree, the  decay  processes  preceding  them  are  due  wholly  to 
bacter  logical  and  allied  influences.  The  organisms  involved 
in  sulfofication  are  probably  many,  including  the  higher  forms 
of  fungi  as  well  as  bacteria.  The  organisms  that  function  in 
the  carbon  cycle  no  doubt  are  active  in  the  sulfur  transfor- 
mations as  well. 

The  possible  sources  of  the  sulfur  which  is  found  in  the 
sulfur  cycle  are  four:  (1)  plant  and  animals  tissue,  (2)  fer- 
tilizers, (3)  rain-water,  and  (4)  the  inorganic  sulfur  of  the 
soil  itself.  The  organic  source  is  probably  the  most  impor- 
tant means  by  which  the  sulfur  supply  of  the  soil  is  aug- 
mented in  practice.  The  addition  of  farm  manure  and  the 
turning  under  of  crop  residues  and  green-manures  will  do 

1  Lipman,  G.  J.,  Suggestions  Concerning  the  Terminology  of  Soil 
Bacteria;  Bot.  Gaz.,  Vol.  51,  pp.  454-460,  1911. 


404        NATURE  AND  PROPERTIES  OF  SOILS 

much  to  retard  the  sulfur  reduction  which  is  constantly  oc- 
curring. Fertilizers,  such  as  acid  phosphate,  ammonium  sul- 
fate, and  potassium  sulfate,  may  also  be  valuable  sources  of 
sulfur.  The  amount  of  sulfur  carried  down  in  rain-water  is 
largely  in  the  sulfate  form  and  is  quite  variable,  ranging  from 
a  few  pounds  of  S03  yearly  to  the  acre  to  over  160  pounds. 
The  rainfall  addition  at  Ithaca,1  New  York,  is  about  65 
pounds  of  S03  to  the  acre  a  year,  while  Stewart 2  reports  a 
yearly  gain  to  the  acre  of  113  pounds  at  the  University  of 
Illinois.  The  inorganic  sulfur  of  the  soil  also  constantly 
tends  to  enter  the  sulfur  cycle  and  must  be  reckoned  with  in 
any  study  of  sulf  ofication. 

222.  Loss  of  sulfur  from  the  soil. — The  loss  of  sulfur 
from  the  soil  under  normal  agricultural  conditions  occurs 
in  two  ways:  (1)  losses  in  drainage,  and  (2)  removal  by 
cropping.  Unless  such  losses  can  adequately  be  met  by  ad- 
ditions of  sulfur  or  sulfur  compounds,  it  is  obvious  that  this 
element  will  become  a  limiting  factor  in  crop  growth.  The 
figures  3  from  the  Cornell  lysimeters  are  very  instructive  in 
this  regard.    The  soil  used  was  a  Dunkirk  silty  clay  loam. 

Table  LXXXIX 

AVERAGE   ANNUAL   LOSS   OF    SULFUR    (as    S08)    BY    PERCOLATION 
AND  CROPPING.     CORNELL  LYSIMETERS.     AVERAGE  OF  10  YEARS. 


Condition 

Pounds  to  the  Acre  of  SO,  Lost  Through 

Drainage 

Crop 

Total 

Bare  soil 

Rotation 

132.0 
108.5 
111.0 

132.0 
149.5 

41.0 
29.2 

Grass 

140.2 

1  Wilson,  B.  D.,  Sulfur  Supplied  to  the  Soil  in  Bain  Water;  Jour. 
Amer.  Soc.  Agron.,  Vol.  13,  No.  5,  pp.  226-229,  1921. 

2 Stewart,  R.,  Sulfur  in  Relation  to  Soil  Fertility:  111.  Agr.  Exp.  Sta., 
Bui.  227,  1920. 

3  Unpublished  data,  Cornell  Agr.  Exp.  Sta.,  Ithaca,  N.  Y. 


SOIL  ORGANISMS  405 

Since  the  sulfur  added  to  the  soil  at  Ithaca,  New  York, 
amounts  to  only  65  pounds  of  S03  yearly  to  the  acre,  other 
sources  of  sulfur  assume  considerable  importance  in  fertility 
practice.  It  seems  probable,  however,  that  the  judicious 
use  of  fertilizers  carrying  sulfur  in  conjunction  with  farm 
manure,  green-manure  and  crop  residues,  will  adequately 
care  for  the  sulfur  needs  of  the  average  soil  (see  par.  264). 

223.  Factors  influencing  sulfofication. — The  sulfofying 
activities  of  the  soil  flora  are  greatly  influenced  by  conditions 
within  the  soil.  Brown  *  has  found  that  the  addition  of  farm 
manure  and  green-manure  greatly  stimulates  sulfofication,  al- 
though carbohydrates  alone  seem  to  exert  a  depressing  influ- 
ence. Lime,  unless  applied  in  very  large  amounts,  encour- 
aged the  transformation  of  the  sulfur  compounds,  increas- 
ing the  amount  of  sulfates  present  in  the  soil.  The  reason 
for  this  influence  is  evident  from  the  reactions  already  quoted. 
The  partial  oxidation  of  hydrogen  sulfide  or  of  free  sulfur 
produces  sulfurous  acid  (H2S03),  which  exerts  a  retarding 
influence  on  further  action,  unless  a  base,  such  as  calcium  or 
magnesium,  is  present  to  form  a  salt  of  this  acid. 

Brown's  results  also  indicate  the  preponderant  influence 
of  aeration,  moisture,  and  organic  matter  on  sulfofication. 
Optimum  conditions  for  crop  growth,  as  far  as  these  factors 
are  concerned,  seem  also  to  be  optimum  for  the  transforma- 
tion of  sulfur  compounds  in  the  soil.  These  same  conditions 
also  favor  satisfactory  reactions  within  the  carbon  cycle  as 
well. 

1  Brown,  P.  E.,  and  Kellogg,  E.  H.,  The  Determination  of  the  Sul- 
fofying Power  of  Soils;  Jour.  Biol.  Chem.,  Vol.  XXI,  No.  1,  pp.  73-89, 
1915. 

Brown,  P.  E.,  and  Johnson,  H.  W.,  Studies  in  Sulfofication ;  Soil  Sci., 
Vol.  I,  No.  4,  pp.  339-362,  1916. 

Brown  determines  the  sulfofying  power  of  soil  by  adding  .1  gram  of 
Na2S  or  free  sulfur  to  100  grams  of  fresh  soil,  adjusting  the  moisture 
content  to  optimum  and  incubating  from  five  to  ten  days.  The  sulfates 
are  then  determined  by  shaking  the  soil  with  water  for  seven  hours, 
filtering  and  precipitating  the  sulfates  with  barium  chloride.  The 
amounts  of  sulfates  are  estimated  in  a  sulfur  photometer.  An  untreated 
sample  of  soil  should  be  run  as  a  check. 


406        NATURE  AND  PROPERTIES  OF  SOILS 

224.  The  sulfur  compost. — It  has  been  noted  by  a  num- 
ber of  experimenters  that  the  presence  of  sulfur  compounds 
in  the  soil  and  especially  elemental  sulfur  tends  to  develop 
considerable  acidity.  The  cause  of  this  acidity  has  already 
been  explained.  In  1916,  Lipman  *  and  his  co-workers  sug- 
gested that  a  practical  use  be  made  of  sulfofication  in  ren- 
dering certain  mineral  nutrients,  such  as  potash  and  phos- 
phoric acid,  available.  Lipman  devised  a  compost  of  sulfur 
and  raw  rock  phosphate.  His  results  seem  to  indicate  that 
sufficient  acid  might  be  formed  by  biological  oxidation  ap- 
preciably to  influence  the  solubility  of  the  rock  phosphate. 

Brown  and  Warner  2  later  used  a  compost  of  sulfur,  farm 
manure  and  raw  rock  phosphate.  Remarkable  increases  in  the 
solubility  of  phosphoric  acid,  measured  by  extraction  with 
a  solution  of  ammonium  citrate,  were  recorded.  The  results 
of  Lipman,  Brown,  and  Warner  have  been  corroborated  by 
Ames  and  Richmond,3  and  Shedd.4  Ames  and  Boltz 5  in 
1919  found  that  sulfur  composted  with  feldspar  appreciably 
influenced  the  solubility  of  potash.  Such  results  as  those 
recorded  above  indicate  the  importance  of  sulfofication  in 
the  soil  under  ordinary  circumstances,  as  well  as  a  possible 
value  in  a  more  intensified  procedure. 

The  practicability  of  using  sulfur  composts  on  the  farm 

1  Lipman,  J.  G.,  et  al.,  Sulfur  Oxidation  in  the  Soil  and  Its  Effects  on 
the  Availability  of  Mineral  Phosphates;  Soil  Sci.,  Vol.  II.  No.  6. 
pp.  499-538,  1916. 

2  Brown,  P.  E.,  and  Warner,  H.  W.,  Production  of  Available  Phos- 
phorus from  Boclc-Phosphate  by  Composting  with  Sulfur  and  Manure; 
Soil  Sci.,  Vol.  IV,  No.  4,  pp.  269-282,  1917. 

3  Ames,  J.  W.,  and  Eichmond,  T.  E.,  Effect  of  Sulfofication  and 
Nitrification  on  Bock  Phosphate;  Soil  Sci.,  Vol.  VI,  No.  4,  pp.  351-364, 
1918.  '  ^  ** 

4  Shedd,  O.  M.,  Effect  of  Oxidation  of  Sulfur  in  Soils  on  the  Solu- 
bility of  Bock-Phosphate  and  on  Nitrification;  Jour.  Aer.  Ees.,  Vol. 
XVIII,  No.  6,  pvp.  329-345,  1919. 

5  Ames,  J.  W.,  and  Boltz,  G.  E.,  Effect  of  Sulfofication  and  Nitrifica- 
tion on  Potassium  and  Other  Soil  Constituents;  Soil  Sci.,  Vol.  VII, 
No.  3,  pp.  183-195,  1919.  See  also,  Tottingham,  W.  E.,  and  Hart,  E.  B., 
Sulfur  and  Sulfur  Composts  in  Belation  to  Plant  Nutrition:  Soil  Sci., 
Vol.  XI,  No.  1,  pp.  49-65,  1921. 


SOIL  ORGANISMS  407 

is  yet  to  be  determined,  and  will  depend  on  a  number  of  fac- 
tors. The  soil  must,  of  course,  be  deficient  in  the  constituent 
composted  with  sulfur.  Otherwise,  an  application  of  sulfur 
alone  would  give  just  as  good  results.  Again  the  cost  of 
composting  must  be  reckoned  with.  It  yet  remains  to  be 
proven  by  crop  growth  whether  the  efficiency  of  sulfur  is  any 
greater  when  it  is  composted  with  such  materials  as  raw  rock 
phosphate  and  farm  manure  and  applied  to  the  soil,  than 
when  these  materials  are  added  separately. 

225.  The  mineral  cycle. — The  strictly  mineral  constitu- 
ents of  the  soil  seem  to  undergo  as  complex  and  intricate 
transformations  as  do  the  elements  that  are  considered  as 
more  closely  related  to  the  soil  organic  matter,  such  as  car- 
bon, nitrogen  and  sulfur.  While  a  part  of  the  mineral  cycle 
is  purely  chemical  or  physico-chemical,  the  biological  phase 
is  by  no  means  unimportant.  In  fact,  were  it  not  for  the  in- 
fluence of  organisms  within  the  soil,  little  or  no  mineral  mat- 
ter, such  as  phosphoric  acid  and  potash,  would  ever  become 
available  to  higher  plants. 

When  plant  or  animal  tissue  enters  the  soil,  it  undergoes 
decay  in  the  manner  already  described,  the  ash  constituents 
being  liberated  and  either  utilized  directly  by  higher  plants 
again  or  converted  into  a  part  of  the  soil  mass.  The  main 
source  of  the  mineral  nutrients  for  any  plant  is  of  course  the 
inorganic  portion  of  the  soil  rather  than  the  organic  part. 
It  is  thus  necessary  to  investigate  what  influence,  if  any,  soil 
organisms  have  on  such  material. 

The  action  of  organisms  on  the  inorganic  portions  of  the 
soil  is  of  two  kinds:  (1)  direct,  and  (2)  indirect.  In  the 
former  the  soil  organisms  themselves  attack  the  mineral  mat- 
ter, rendering  part  of  it  available.  Some  of  this  soluble  ma- 
terial is  absorbed  by  the  organisms,  becoming  a  part  of  the 
cell  contents.  When  the  fungus  or  bacterium  dies,  this  ma- 
terial through  decay  again  becomes  available  and  may  be 
used  by  higher  plants.     While  most  soil  organisms  probably 


408        NATURE  AND  PROPERTIES  OF  SOILS 

function  to  a  certain  extent  in  this  direction,  some  are  es- 
pecially active.  It  is  known  that  B.  mycoides,  B.  mesenteric 
cus  and  B.  megatherium  are  capable  of  assimilating  phos- 
phorus in  considerable  quantities,  while  such  organisms  as 
Beggiotoa  and  OpMdomonas  store  up  sulfur  in  large  amounts. 
In  the  same  way  iron,  potassium,  calcium,  and  like  elements 
may  be  utilized.  While  such  biological  action  is  at  the  time 
a  direct  competition  with  higher  plants,  more  mineral  ma- 
terial is  ultimately  available  in  the  soil  through  such  activ- 
ities. 

While  the  direct  effects  of  organisms  on  soil  minerals  is 
no  doubt  very  important,  the  direct  influences  seem  to  be 
more  vital  in  a  practical  way.  While  this  indirect  influence 
may  be  in  part  enzymic,  it  is  probably  largely  due  to  the 
production  of  carbon  dioxide,  which  accompanies  all  types  of 
life  processes.  The  sulfurous  acid  and  nitrous  acid  of  the 
sulfur  and  the  nitrogen  cycles,  respectively,  are  also  active 
to  a  certain  extent.  The  preceding  discussion  of  the  sulfur 
compost  indicates  how  vigorous  the  biological  oxidation  with- 
in the  sulfur  cycle  may  become  under  certain  conditions.  In 
the  soil,  however,  carbon  dioxide  is  probably  by  far  the  most 
important.1  Since  the  significance  of  carbon  dioxide  has 
already  been  adequately  discussed  (pars.  17,  58  and  132),  it  is 
sufficient  at  this  point  to  state  that  this  gas,  because  of  its 
large  amounts  and  its  intimate  relationship  to  the  mineral 
material,  is  probably  the  most  effective  solvent  agent  in  the  soil. 

1  Typical  reactions  involving  tri-calcium  phosphate,  orthoclase  and  cal- 
cium carbonate  are  as  follows: 

Ca,(P04)a  +  2C02  +  2HaO  =  Ca2H2(P04)2  +  Ca(HC03)2. 
2KAlSi308  +  C02  +  2H20  =  H4Al,SiaO,  +  KaCO.  +  4SiOa. 
CaCG3  +  H20  +  COa  =  Ca(HCO,)a. 


CHAPTER  XXI 
SOIL  ORGANISMS— THE  NITROGEN  CYCLE 

Of  the  various  nutrient  materials  applied  to  the  soil  for 
the  use  of  plants  nitrogen  has  the  highest  commercial  value 
and  is  absorbed  in  very  large  quantities.  Moreover,  nitro- 
gen is  lost  from  the  soil  in  considerable  amounts  in  drainage 
water  and  possibly  to  some  extent  in  gaseous  form.  The 
great  importance  of  this  element  and  of  its  compounds  in 
agriculture  and  the  possibility  of  it  becoming  a  limiting  factor 
in  crop  production  has  le^d  to  much  study  regarding  its  re- 
actions and  movements  in  the  soil. 

The  original  source  of  the  world 's  supply  of  combined  nitro- 
gen has  been  the  atmosphere  and,  as  the  free  gas  is  exceed- 
ingly inert,1  the  natural  forces  which  facilitate  its  combina- 
tion must  be  extremely  powerful.  The  movement  of  nitrogen 
from  air  to  soil,  from  soil  to  plant,  from  plant  back  to  soil  or 
to  animal,  and  from  animal  to  soil,  with  a  return  to  air  at 
various  stages,  involves  many  forces,  many  factors,  many  or- 
ganisms, and  many  reactions.  These  complicated  changes 
are  spoken  of  as  the  nitrogen  cycle. 

226.  The  nitrogen  cycle. — In  tracing  the  various  trans- 
formations through  which  the  nitrogen  passes,  the  conspicu- 
ous feature  is  the  great  complexity  of  the  cycle.  Apparently 
the  nitrogen  cycle  is  much  more  extended  and  intricate  than 
either  the  carbon  or  sulfur  cycles.    This  complexity,  however, 

1  Because  nitrogen  is  such  an  inert  gas,  it  must  not  be  inferred  that  it 
forms  inactive  compounds  with  other  materials.  In  combination  it  is 
extremely  active,  seemingly  being  the  basis  of  all  plant  and  animal  life 
processes. 

409 


410        NATURE  AND  PROPERTIES  OF  SOILS*  ..  „  ^ 

is  more  apparent  than  real.  The  transformation  of  nitro- 
gen has  received  so  much  attention  and  study  that  more 
is  known  regarding  the  changes  involved.  The  other  cycles 
are  probably  just  as  extended  and  complicated,  the  lack  of 
knowledge  forcing  a  simpler  presentation. 

From  the  standpoint  of  soil  fertility  the  compounds  that 
are  produced  in  the  nitrogen  cycle  and  the  relation  of  these 
materials  to  plant  growth  are  of  major  consideration.  While 
the  organisms  involved  in  the  transformation  should  receive 
as  much  attention  as  is  practicable,  the  approach  should  be  by 
means  of  biological-chemistry  rather  than  through  bacteri- 
ology. 

It  must  not  be  inferred  that  the  carbon,  sulfur  and  nitro- 
gen cycles  are  distinct  or  that  transformations  may  proceed 
in  one  with  no  activity  in  the  others.  As  a  matter  of  fact, 
the  cycles  are  interlocked  in  a  hopelessly  intricate  manner. 
The  decomposition  of  proteid  matter  involves  all  of  the  cycles 
already  mentioned.  The  carbon,  sulfur,  and  nitrogen  un- 
dergo distinctly  different  transformations,  but  the  changes 
are  so  closely  related  as  to  make  definite  lines  of  distinction 
very  difficult.  Proteid  matter  may  produce  urea,  carbon 
dioxide,  water,  and  sulfates.  Certain  of /these  products  often 
strongly  influence  the  solubility  of  the  soil  minerals.  Thus, 
the  four  cycles  already  mentioned  would  be  involved  in  the 
decomposition  of  one  original  compound. 

227.  Decay  and  putrefaction.1 — The  decomposition  of 
most  nitrogenous  matter  is  very  rapid  in  a  normal  soil,  the 
putrefactive  influences  producing  partially  decayed  sub- 
stances of  great  variety.2  Some  of  these  materials  are  very 
complicated,  while  others  are  capable  of  being  absorbed  di- 

1  Decomposition  and  decay  are  general  terms,  referring  to  all  types 
of  biological  degradation.  Fermentation  refers  to  the  decomposition  "of 
carbohydrates,  while  putrefaction  has  to  do  with  nitrogenous  materials. 
The  two  latter  terms  are  generally  very  loosely  used. 

JLathrop,  E.  C,  Protein  Decomposition  in  Soils:  Soil  Sci.,  Vol.  I, 
No.  6,  pp.  509-532,  1916. 


SOIL  ORGANISMS  411 


rectly  by  plants  without  further  change.  Carbon  dioxide  and 
water  are  formed  continuously  as  the  process  advances.  The 
sulfur  of  the  proteid  compounds  produces  hydrogen  sulfide 
or  free  sulfur  and  later  sulfates. 

Hutchinson  and  Miller,1  as  well  as  other  investigators, 
have  studied  the  question  of  the  assimilation  of  nitrogenous 
organic  compounds  by  higher  plants.  The  general  conclu- 
sions indicate  that  such  a  source  of  nitrogen  is  quite  impor- 
tant and  sometimes  allows  the  plant  to  benefit  markedly  from 
the  assimilation  of  such  materials.  Maize,  for  example,  seems 
to  be  particularly  stimulated  by  farm  manure,  which  carries 
large  amounts  of  organic  nitrogenous  compounds  such  as 
urea.  Acetamide,  urea,  barbituric  acid,  creatinine,  alloxan, 
peptone,  and  a  number  of  other  organic  compounds  have 
been  shown  to  be  available  to  certain  higher  plants. 

Decay  and  putrefaction  are  carried  on  by  a  large  number 
of  organisms,  the  higher  fungi  as  well  as  such  bacteria  as 
B.  subtilis,  B.  myc&ides,  and  similar  micro-organisms  engag- 
ing in  the  decomposition  processes.  Some  of  the  charac- 
teristic, although  not  constant,  products  formed  in  the  pu- 
trefaction of  albumin  and  proteins  are  albumoses,  peptones, 
and  amino  acids,  followed  by  the  formation  of  cadaverine, 
putrescine,  skatol,  and  indol.  Where  an  abundant  supply 
of  oxygen  is  present,  or  where  a  sufficient  supply  of  carbo- 
hydrates exists,  the  latter  substances  are  not  formed.  There 
are  many  other  products  of  putrefaction,  including  a  num- 
ber of  gases,  as  carbon  dioxide,  hydrogen  sulfide,  marsh  gas, 
phosphine,  hydrogen,  nitrogen,  and  the  like. 

Present-day  knowledge  of  the  subject  does  not  make  it  pos- 
sible to  present  a  list  of  the  organisms  concerned  in  each  step, 
or  to  name  all  the  intermediate  products  formed.  For  the 
student  of  the  soil  the  first  consideration  is  a  knowledge  of 

1  Hutchinson,  H.  B.,  and  Miller,  N.  H.  J.,  The  Direct  Assimilation  of 
Inorganic  and  Organic  Forms  of  Nitrogen  by  Higher  Plants;  Centrlb.  f. 
Bakt.,  II,  Band  30,  Seite  513-547,  1911. 


412        NATURE  AND  PROPERTIES  OF  SOILS 

the  circumstances  under  which  the  nitrogen  is  made  avail- 
able to  plants,  and  the  conditions  that  are  likely  to  encourage 
its  loss  from  the  soil. 

228.  Ammonification  may  be  considered  as  the  second  step 
in  the  simplification  which  nitrogenous  compounds  undergo 
in  the  soil.  As  the  name  implies,  it  is  the  stage  of  the  decay 
process  in  which  ammonia  is  one  of  the  important  products. 
Like  other  processes  of  decomposition,  there  are  many  species 
of  organisms  capable  of  producing  ammonia,  the  higher  fungi 


Fig.  58. — Some  soil  organisms  important  in  the  nitrogen  cycle,  (a) 
Azotobacter  agilis;  (b)  nitrate  bacteria.  Urea  bacteria,  (c)  Uro- 
bacillus  miguelii  and  (d)  Urobacillus  leubii. 

and  algae  as  well  as  bacteria  participating  in  the  change  in  the 
character  of  the  nitrogen  compounds. 

Different  soil  organisms  display  diverse  abilities  in  con- 
verting the  nitrogen  of  the  same  organic  material  into  am- 
monia, some  acting  more  rapidly  or  more  thoroughly  than 
others.  In  tests  by  certain  investigators  in  which  the  same 
bacteria  were  allowed  to  act  on  different  substances,  the  order 
of  their  efficiency  was  reversed  with  a  change  of  substance. 
This  characteristic  preference  of  a  class  of  organisms  for  the 
decomposition  of  certain  substances  is  made  evident  by  the 
experiments  of  Sackett,1  who  found  that  in  some  soils  dried 

1Sackett,  W.  G.,  The  Ammonifying  Efficiency  of  Certain  Colorado 
Soils;  Colo.  Agr.  Exp.  Sta.,  Bui.  184,  1912. 


SOIL  ORGANISMS  413 

blood  was  ammonified  more  rapidly  than  was  cottonseed  meal, 
while  in  other  soils  the  reverse  was  true. 

While  the  soil  fungi  have  been  but  little  studied,  the  litera- 
ture available  seems  to  indicate  that  they  take  an  important 
part  in  all  soil  processes,  except  possibly  the  fixation  of  at- 
mospheric nitrogen  and  the  formation  of  nitrates.  Most  soil 
fungi  produce  ammonia  readily.  Waksman x  found  such 
forms  as  Mucor  racemosus,  Pencillium  lilacinum,  and  Bhiz- 
opus  sp.  II  compared,  favorably  in  capacity  to  produce  am- 
monia with  Bacillus  mycoides  when  grown  in  artificial  cul- 
ture, blood  and  cottonseed  meal  being  the  sources  of  nitrogen. 
Kopeloff 2  found  that  certain  fungi  seemed  to  prefer  an  acid 
medium  for  their  ammonifying  activities.  This  suggests  that 
a  natural  provision  is  thus  made  for  ammonification,  no  mat- 
ter what  the  soil  reaction  may  be. 

Among  the  bacteria  producing  ammonification  are  B.  my- 
coides, B.  subtilis,  B.  mesentericus  vulgatus,  B.  janthinus, 
and  B.  proteus  vulgaris.  Of  these,  B.  mycoides  has  been  very 
carefully  studied,  and  the  findings  of  Marchal 3  may  be  taken 
as  representative  of  the  process  of  ammonification.  He  found 
that  when  this  bacterium  was  seeded  on  a  neutral  solution  of 
albumin,  ammonia  and  carbon  dioxide  were  produced,  to- 
gether with  small  amounts  of  peptone,  leucine,  tyrosine,  and 
formic,  butyric,  and  propionic  acids.  He  concludes  that  in 
the  process  atmospheric  oxygen  is  used,  and  that  the  carbon 
of  the  albumin  is  converted  into  carbon  doxide,  the  sulfur 
into  sulfates,  and  the  hydrogen  partly  into  water,  and  partly 
into  ammonia  by  combining  with  the  nitrogen  of  the  organic 

1  Waksman,  S.  A.,  Soil  Fungi  and  Their  Activities;  Soil  Sci.,  Vol. 
II,  No.  2,  pp.  103-155,  1916.  See  also,  McLean,  H.  C,  and  Wilson, 
G.  W.,  Ammonification  Studies  with  Soil  Fungi;  N.  J.  Agr.  Exp.  Sta., 
Bui.  270,  1914. 

a  Kopeloff,  N.,  The  Effect  of  Soil  Beaction  on  Ammonification  by 
Certain  Soil  Fungi;  Soil  Sci.,  Vol.  I,  No.  6,  pp.  541-573,  1916. 

3  Marchal,  E.,  Sur  la  Production  de  VAmmoniaque  dans  le  Sol  par 
les  Microbes;  Bulletins  de  PAcad.  Koyale  de  Belg.,  3  series,  T.  25,  pp. 
727-776  j   1893. 


414        NATURE  AND  PROPERTIES  OF  SOILS 

substance.  Marchal  found  that  B.  mycoides  was  also  capable 
of  ammonifying  casein,  fibrin,  legumin,  glutin,  myosin,  serin, 
peptones,  creatine,  leucine,  tyrosine,  and  asparagine,  but 
not  urea. 

The  following  reactions  may  be  cited  as  indicating  the 
changes  that  probably  occur  when  albumin  and  urea  undergo 
ammonification : 

C72H112N18S022  +  7702  =  29H20  +  72C02  +  S03  +  18NH3 
Albumin 

CON2H4  +  2H20  =  (NH4)2C03 
Urea 

While  ammonification  x  seems  to  proceed  to  the  best  advan- 
tage in  a  well-drained  and  aerated  soil  with  plenty  of  active 
basic  material  present,  it  will  take  place  to  some  extent  under 
almost  any  condition,  due  to  the  great  number  of  different  or- 
ganisms capable  of  accomplishing  the  change.  In  certain 
soils,  as  shown  by  Russell  and  Hutchinson  2  as  well  as  by 
other  authors  (see  par.  211),  protozoa  may  retard  ammoni- 
fication by  feeding  on  the  chief  ammonia-producing  organ- 
isms.    Such  a  condition  is  seldom  serious  in  arable  soils. 

1  The  ammonifying  efficiency  of  a  soil  is  usually  determined  by  treat- 
ing a  200-gram  sample  of  fresh  soil  with  cottonseed  meal  or  dried  blood 
carrying  120  milligrams  of  nitrogen.  The  mixture  is  then  incubated, 
usually  for  seven  days,  at  optimum  temperature  and  moisture.  The  in- 
crease in  ammonia  is  taken  as  a  measure  of  the  ammonifying  efficiency. 
The  artificial  nature  of  the  test  detracts  largely  from  its  value.  See 
Temple,  J.  C,  The  Value  of  Ammonification  Tests;  Ga.  Agr.  Exp.  Sta., 
Bui.  126,  1919. 

2RuSsell,  E.  J.,  and  Darbishire,  F.  V.,  Oxidation  in  Soils  and  Its 
Relation  to  Productiveness.  Part  2.  The  Influence  of  Partial  Steriliza- 
tion; Jour.  Agr.  Sci.,  Vol.  2,  pp.  305-326,  1907. 

Russell,  E.  J.,  and  Hutchinson,  H.  B.,  The  Effect  of  Partial  Steriliza- 
tion of  Soil  on  the  Production  of  Plant  Food;  Jour.  Agr.  Sci.,  Vol.  3, 
pp.  111-144,  1909. 

Russell,  E.  J.,  and  Hutchinson,  H.  B.,  The  Limitation  of  Bacterial 
Numbers  in  Normal  Soils  and  Its  Consequences ;  Jour.  Agr.  Sci.,  Vol. 
5,  pp.  152-221,  1903. 

Buddin,  W.,  Partial  Sterilization  of  Soil  by  Volatile  and  Non- 
volatile Antiseptics;  Jour.  Agr.  Sci.,  Vol.  6,  pp.  417-451,  1914. 


SOIL  ORGANISMS  415 

229.  Nitrification. — Some  agricultural  plants  can  utilize 
ammonium  salts  as  a  source  of  nitrogen.1  This  has  been  shown 
to  be  true  for  rice,  maize,  peas,  barley,  and  potatoes  (see  par. 
248).  Most  plants,  however,  except  for  rice,  show  a  decided 
preference  for  nitrogen  in  the  nitrate  form.  Whether  these 
common  crops  can  thrive  as  well  on  ammonium  salts  as  on 
nitrates  has  not  been  definitely  demonstrated.  In  most  arable 
soils  the  transformation  of  nitrogen  does  not  stop  with  its 
conversion  into  ammonia,  but  goes  on  by  an  oxidation  proc- 
ess to  the  formation  of  nitrous  acid.  The  nitrous  acid,  after 
reaction  with  a  base,  is  farther  oxidized,  a  salt  of  nitric  acid 
resulting.  This  process  of  oxidation  is  generally  spoken  of 
as  nitrification.  The  reactions  involved  may  be  written  as 
follows : 

2NH3  +  302  =  2HN02  +  2H20  2 

2HN02  +  CaH2(C03)2  =  Ca(NO,)2  +  2H20  +  2C02  3 

Ca(N02)2  +  02  =  Ca(N03)2 

Each  of  these  steps  is  brought  about  by  a  distinct  bacteri- 
um, but  the  groups  are  closely  related.  Collectively  they  are 
called  nitrobacteria.  Nitrosomonas  and  Nitrosococcus  are 
the  bacteria  concerned  in  the  conversion  of  ammonia  into 
nitrous  acid  or  nitrites.  The  former  are  supposed  to  be  char- 
acteristic of  European,  and  the  latter  of  American,  soils. 
The  organisms  concerned  in  the  oxidation  of  nitrites  to  ni- 

1Kelley,  W.  P.,  The  Assimilation  of  Nitrogen  by  Bice;  Haw.  Agr. 
Exp.  Sta.,  Bui.  24,  1911. 

Hutchinson,  H.  B.,  and  Miller,  N.  H.  J.,  The  Direct  Assimilation  of 
Inorganic  and  Organic  Forms  of  Nitrogen  by  Higher  Plants;  Centrlb. 
f.  Bakt.,  II,  Band  30,  Seite  513-547,  1911. 

2Loew  states  that  the  reaction  is  as  follows: 

2NH3  +  202  =  2HN02  +  4H 

Loew,  O.,  Die  Chemischen  Verhaltnisse  des  BaJcterienlebens:  II. 
Centrlb.  f.  Bakt.,  II,  Bd.  9,  Seite  690-697,  1891. 

3  It  has  often  been  suggested  that  the  acid  produced  by  the  nitrifying 
process  is  of  considerable  importance  in  rendering  mineral  nutrients 
available.  While  this  may  be  true,  the  extent  to  which  the  solution 
phenomenon  takes  place  and  its  practical  significance  have  never  been 
satisfactorily  established  by  experimentation. 


416        NATURE  AND  PROPERTIES  OF  SOILS 

trates  are  generally  designated  as  Nitrobacter.  In  practice 
these  bacteria  are  generally  spoken  of  as  nitrite  and  nitrate 
Organisms.1  The  conditions  favoring  the  two  groups  are 
practically  the  same.  As  a  consequence,  nitrification  is  gen- 
erally discussed  as  though  the  transformation  was  only  one 
step  and  depended  on  one  group  of  organisms.2 

Just  as  ammonification  follows  closely  on  putrefaction,  so 
nitrification  closely  accompanies  the  production  of  ammonia. 
In  fact,  the  processes  are  so  well  synchronized  in  a  normal 
soil  that  only  traces  of  ammonia  and  nitrites  are  usually 
found.  The  nitrates,  however,  may  accumulate  in  large 
amounts. 

Marked   differences   have   been   noted    in   the   nitrifying 3 

1  While  it  was  known  from  the  middle  of  the  nineteenth  century  that 
nitrogenous  compounds  added  to  the  soil  quickly  produced  nitrates,  it 
was  not  until  1878  that  Schloessing  and  Miintz  demonstrated  that  the 
process  was  biological.  In  1890  Winogradsky  succeeded  in  isolating 
the  organisms.  As  they  do  not  develop  on  ordinary  medium,  as  do 
the  decay  and  ammonifying  bacteria,  a  special  technique  was  necessary. 
Winogradsky  used  silicic-acid-gel  plates  containing  certain  inorganic 
salts,  as  he  found  that  the  presence  of  even  small  amounts  of  organic 
matter  prevented  the  development  of  the  organisms.  In  the  soil,  how- 
ever, well-decayed  organic  matter  generally  stimulates  rather  than  de- 
presses nitrification.  For  a  review  of  literature  and  methods  of  isolat- 
ing nitrifying  organisms,  see  Gibbs,  W.  M.,  The  Isolation  and  Study*  of 
Nitrifying  Bacteria;  Soil  Sci.,  Vol.  VIII,  No.  6,  pp.  427-471,  1919. 

aKaserer  has  isolated  an  organism,  which  he  called  B.  Nitrator,  that 
can  oxidize  ammonia  directly  to  nitrate.  He  writes  the  reaction  as 
follows : 

NH3  +  H2CO,  +  Oa  =  HN03  +  H20  +  CH2a 

He  thinks  that  the  energy  necessary  for  the  completion  of  the  reac- 
tion is  obtained  from  the  formaldehyde    (CH20)    as  follows: 
CH20  +  02  =  H20  +  C02  +  Energy 

The  correlation  between  carbon  dioxide  production  and  nitrate  accumu- 
lation lends  probability  to  this  theory. 

Kaserer,  H.,  On  Some  New  Nitrogen  Bacteria  with  Autotrophic  Habits 
of  Life;  Noted  in  Exp.  Sta.  Eecord,  Vol.  18,  p.  534,  1905-1906. 

'  The  nitrifying  efficiency  of  a  soil  is  usually  determined  by  treating 
a  100-gram  sample  held  in  a  tumbler  with  a  suitable  amount  of  ammonia 
sulfate  or  some  other  readily  nitrifiable  material.  After  incubation  for 
a  suitable  period  at  optimum  temperature  and  moisture,  the  increase  of 
nitrate  nitrogen  is  determined.  This  method  is  merely  comparative  and 
measures  only  the  nitrate  accumulation.  Its  value  is  limited  as  it  does 
not  simulate  field  conditions. 


SOIL  ORGANISMS 


417 


power  of  different  soils.  Highly  productive  soils  have  gen- 
erally been  found  to  maintain  a  greater  nitrifying  efficiency 
than  less  productive  soils,  but  this  is  not  always  the  case, 
as  factors  other  than  available  nitrogen  may  limit  the  pro- 
ductiveness of  a  soil. 

With  the  formation  of  nitrate  nitrogen,  the  main  portion 
of  the  nitrogen  cycle  is  completed,  since  plants  absorb  most 
of  their  nitrogen  as  the  nitrate  ion.     Of  this  cycle,  from 


ANIMAL 


MANURE 

DECAY 

PARTIALLY    DECAYED 
^  COMBOS 

.AMMONIFICATION 

AMMONIA 


=1CAT10N|    sJ 

-^-NITRITES'*/ 


NITRIFICATION 
NITRATES  -#.NITR 

Fig.  59. — Diagram  representing  the  movements  of  nitrogen  between 
soil,  plants,  animals  and  the  atmosphere.  These  transformations 
are  termed  the  "nitrogen  cycle.' ' 


plant  to  soil,  and  from  soil  to  plant  again,  the  nitrification  re- 
action is  the  weakest  point,  since  the  other  biological  changes 
proceed  to  a  certain  extent  in  spite  of  unfavorable  soil  con- 
ditions. Nitrification  is  easily  retarded  and  may  even  be 
brought  to  a  standstill.  As  a  consequence,  the  factors  affect- 
ing this  particular  portion  of  the  nitrogen  cycle  are  of  special 
interest.  A  soil  favorable  to  nitrification  is  generally  wholly 
favorable  to  the  other  desirable  processes  involving  nitrogen 
transformations. 


418        NATURE  AND  PROPERTIES  OF  SOILS 

230.  Relation  of  soil  conditions  to  nitrification. — Al- 
though a  very  great  number  of  factors  influence  the  process 
of  nitrification,  the  principal  controls  may  be  listed  as  fol- 
lows: (1)  presence  of  nitrifiable  substance,  (2)  aeration,  (3) 
temperature,  (4)  moisture,  (5)  soil  reaction,  and  (6)  the 
presence  of  soluble  salts. 

A  peculiarity  in  the  artificial  cultivation  of  nitrifying  bac- 
teria is  that  they  cannot  be  grown  in  artificial  media  con- 
taining organic  matter.  In  the  soil,  however,  organic  matter, 
when  well  decayed,  stimulates  nitrification,1  provided  aera- 
tion and  other  conditions  are  favorable  (see  par.  313).  The 
application  of  twenty  tons  of  farm  manure  to  the  acre  to  sod 
on  a  clay  loam  soil  for  three  consecutive  years,  at  Cornell 
University,2  resulted  in  a  larger  accumulation  and  probably 
a  larger  production  of  nitrates  on  the  manured  soils  than 
on  a  contiguous  plat  of  similar  soil  left  unmanured.  This 
was  especially  true  during  the  third  year  of  the  application, 
when  the  land  was  in  sod,  and  also  during  the  fourth  year, 
when  no  manure  was  applied  to  either  plat  and  when  both 
plats  were  planted  to  maize,  as  may  be  seen  from  Table  XC 
(page  419). 

These  data  indicate  not  only  a  marked  influence  of  organic 
matter  on  nitrification  but  also  an  effect  from  aeration.  Even 
allowing  for  a  direct  and  differential  influence  on  nitrifica- 
tion by  the  two  crops,  it  is  evident  that  tillage  is  a  factor. 
Further  experimental  data  from  Cornell  University  may  be 
quoted.  Columns  of  soil  eight  inches  in  diameter  and  eight 
inches  in  depth  were  removed  from  a  field  of  clay  loam  on 
the  Cornell  University  farm  and  carried  to  the  greenhouse 
without  disturbing  the  original  structure  of  the  soil.  At 
the  same  time,  vessels  of  similar  size  were  filled  with  soil  dug 
from  a  spot  near  by.     These  may  be  termed  unaerated  and 

The   turning   under   of   a   green-manuring   crop    generally    depresses 
nitrification  at  first.    Once  the  decay  process  is  well  under  way,  nitrifica- 
tion activities  seem  to  be  stimulated. 
2  Unpublished  data.    Cornell  Agr.  Exp.  Sta.,  Ithaca,  N.  Y. 


SOIL  ORGANISMS 


419 


Table  XC 

NITRATE    ACCUMULATION    ON    HEAVILY    MANURED    AND    ON    UN- 
MANURED  SOIL. 


N03  in  Parts  to  a  Million  of 
Dry  Soil 

Crop 

Unmanured 
Soil 

Twenty  Tons 

Manure  to  the 

Acre  for  Three 

Years 

Timothy  (3rd  year) 

April   23rd 

June  13th 

8.2 

.8 

1.8 

17.5 

50.0 

151.0 

21.0 
1.1 

August  14th 

3.0 

Maize  following  timothy 

May  19th 

July  6th 

August  10th 

20.1 
105.0 
184.0 

aerated  soils.  Both  were  kept  at  the  same  temperature  and 
moisture  content  in  the  greenhouse,  but  no  plants  were  grown 
on  them.    The  accumulation  of  nitrates  was  as  follows: 


Table  XCI 


Time  of  Analysis 

Nitrates,  Parts  per  Million  Dry 
Soil 

Unaerated  Soil 

Aerated  Soil 

When  taken  from  field. . .  . 
After  standing  one  month.  . 
After  standing  two  months. 

3.2 
4.2 

9.0 

3.2 
17.6 
45.6 

It  has  often  been  assumed  that  carbon  dioxide  is  a  detri- 
mental factor  in  biological  activity  in  two  respects:  by  the 
replacement  of  oxygen  and  by  a  toxic  influence  on  the  organ- 


420        NATURE  AND  PROPERTIES  OF  SOILS 

isms.  Recent  experimentation,1  however,  indicates  that  car- 
bon dioxide  has  little  or  no  effect  on  nitrification  and  am- 
monification  as  long  as  appreciable  quantities  of  oxygen  are 
present.  Aeration,  insofar  as  most  biological  activities  are 
concerned,  has  to  do  more  with  the  presence  of  oxygen  than 
the  elimination  of  the  carbon  dioxide  which  is  always  form- 
ing. 

Since  aeration  is  such  a  factor  in  nitrification,  the  trans- 
formation is  very  largely  confined  to  the  surface  layers  of 
soil,  except  in  the  rich  and  porous  subsoils  of  arid  and  semi- 
arid  regions.  The  lack  of  nitrate  formation  in  the  lower 
depths  is  probably  influenced  by  temperature  as  well  as  by 
lack  of  oxygen  and  organic  matter.  At  5°  C.  nitrification  is 
very  feeble.  The  optimum  temperature  seems  to  range  from 
25°  to  30°  C.  The  drainage  of  a  soil  probably  promotes  nitri- 
fication quite  as  much  by  facilitating  a  rise  of  temperature 
as  by  promoting  the  entrance  of  oxygen,  especially  in  the 
spring. 

The  speed  with  which  nitrification  proceeds  in  a  soil  is 
governed  to  a  marked  extent  by  water  content,2  the  process 
being  retarded  by  both  low  and  high  moisture  conditions. 
In  practice,  it  is  safe  to  assume  that  the  optimum  moisture 
as  recognized  for  higher  plants  is  optimum  for  nitrification 

Glummer,  J.  K.,  Some  Effects  of  Oxygen  and  Carbon  Dioxide  on 
Nitrification  and  Ammonification  in  Soils;  Cornell  Agr.  Exp.  Sta.,  Bui. 
384,  1916. 

2 Coleman,  L.  C,  Untersuchungen  iiber  Nitrification;  Centbl.  f.  Bakt., 
II,  Bd.  20,  Seite  401-420  and  484-513,  1908. 

Fraps,  G.  S.,  The  Production  of  Active  Nitrogen  in  the  Soil;  Tex.  Agr. 
Exp.  Sta.,  Bui.  106,  1908. 

Patterson,  J.  W.,  and  Scott,  P.  K.,  The  Influence  of  Soil  Moisture 
upon  Nitrification;  Jour.  Dept.  Agr.,  Victoria,  Vol.  10,  pp.  275-282, 
1912.  ** 

Stewart,  B.,  and  Greaves,  J.  E.,  The  Production  and  Movement  of 
Nitric  Nitrogen  in  Soil;  Centbl.  f.  Bakt.,  II,  Bd.  34,  Seite  115-147, 
1912.  ' 

Gainey,  P.  L.,  The  Effect  of  Time  and  Depth  of  Cultivating  a\ 
Wheat  Seed-bed  Upon  Bacterial  Activity  in  the  Soil;  Soil  Sci.,  Vol. 
II,  No.  2,  pp.  193-204,  1916. 


SOIL  ORGANISMS  421 

also.  Greaves  and  Carter  x  found  that  a  moisture  content  of 
about  55  per  cent,  of  the  water-holding  capacity,  as  determined 
by  the  Hilgard  method  (see  par.  90),  was  especially  favorable 
for  nitrification. 

It  has  generally  been  considered  that  nitrification  was  very 
much  retarded  if  not  actually  brought  to  a  standstill  in  an 
acid  soil.2  Recent  data,3  however,  seem  to  indicate  that  the 
process  will  proceed  in  acid  soil,  although  the  addition  of 
lime  in  some  form  is  usually  beneficial.  The  marked  stimula- 
tion of  liming  to  certain  crops  may  be  due  partially  to  the  in- 
fluence of  the  lime  on  the  nitrifying  organisms.  This  rela- 
tionship should  be  particularly  noticeable  if  the  crop  in 
question  is  unable  to  utilize  organic  or  ammoniacal  forms  of 
nitrogen. 

The  influence  of  certain  mineral  salts  is  quite  significant.4 
Small  amounts  of  salts,  even  those  of  manganese,  stimulate 
the  process.  Sodium  nitrate,  unless  applied  in  excessive 
amounts,  promotes  the  nitrification  of  dried  blood  and  cotton- 
seed meal.  In  general,  the  stimulation  of  soil  bacteria  by  the 
application  of  fertilizer  salts  is  coordinate  with  the  stimula- 
tion ordinarily  observed  in  higher  plants.  Rational  fertilizer 
practice,  therefore,  promotes  nitrification,  and  no  important 
retarding  influences  may  be  expected  on  bacterial  activity 
unless  the  crop  is  itself  directly  injured. 

1  Greaves,  J.  E.,  and  Carter,  E.  G.,  Influence  of  Moisture  on  the 
Bacterial  Activities  of  the  Soil;  Soil  Sci.,  Vol.  X,  No.  5,  pp.  361-387, 
1920. 

2  Hall,  A.  D.,  Fertilisers  and  Manure,  pp.  62-64,  New  York,  1909. 

3  Temple,  J.  C,  Nitrification  in  Acid  or  Non-basic  Soils;  Ga.  Agr. 
Exp.  Sta.,  Bui.  103,  1914. 

White,  G.  W.,  Nitrification  in  Eelation  to  the  Eeaction  of  the  Soil; 
Penn.  Agr.  Exp.  Sta.,  Ann.  Rep.  1913-14,  pp.  70-84,  1916. 

*Kelley,  W.  P.,  Nitrification  in  Semiarid  Soils;  Jour.  Agr.  Ees., 
Vol.  VII,  No.  10,  pp.  417-437,  1916. 

Brown,  P.  E.,  and  Hitchcock,  E.  B.,  The  Effect  of  AlJcali  Salts  on 
Nitrification;  Soil  Sci.,  Vol.  IV,  No.  5,  pp.  207-229,  1917. 

Brown,  P.  E.,  and  Minges,  G.  A.,  The  Effect  of  Some  Manganese 
Salts  on  Ammonification  and  Nitrification;  Soil  Sci.,  Vol.  II,  No.  1, 
pp.  67-85,  1916. 


422        NATURE  AND  PROPERTIES  OF  SOILS 

231.  Influences  of  higher  plants  on  nitrification.— It  has 
been  known  for  some  time  that  the  nitrate  content  of  a  soil 
varies  with  the  crop  that  occupies  the  land.  King  and  Whit- 
son  x  reported  in  1901  that  the  accumulation  of  nitrates  was 
greatest  under  maize,  with  potatoes  next  and  alfalfa  and 
clover  much  lower.  Stewart  and  Greaves,2  in  an  experiment 
covering  several  years,  also  found  that  maize  allowed  the  great- 
est accumulation,  with  potatoes,  oats,  and  alfalfa  following 
in  the  order  named.  Brown  and  Maclntire 3  report  forty 
times  more  nitrates  in  a  soil  cropped  to  maize  than  when 
planted  to  grass.  As  the  moisture  content  was  practically 
the  same  in  each  case,  the  difference  cannot  be  ascribed  to 
this  influence. 

Perhaps  the  most  extensive  work  along  this  line  is  that  of 
Lyon  and  Bizzell.4  They  noted  a  characteristic  relationship 
between  the  crop  at  different  stages  of  growth  and  the  cor- 
responding nitrate  content  of  the  soil.  During  the  most  ac- 
tive growing  period  of  maize,  although  the  crop  was  absorb- 
ing nitrogen  in  large  amounts,  the  nitrates  were  frequently 
higher  under  the  maize  than  in  a  contiguous  fallow  plat.  Oat 
land  contained  less  nitrates,  while  grass  seemed  to  retard 
markedly  the  accumulation  of  nitrates.  Whether  the  nitrate 
organisms  are  stimulated  by  certain  plants  or  whether  nitrate 
formation  is  merely  depressed  more  by  some  plants  than  by 
others  is  not  known.  It  is  clear,  however,  that  the  relation- 
ship of  crop  to  nitrification  must  be  reckoned  with  in  practical 

1King,  F.  H.,  and  Whitson,  A.  E.,  Development  and  Distribution  of 
Nitrates  and  Other  Soluble  Salts  in  Cultivated  Soils;  Wis.  Agr.  Exp. 
Sta.,  Bui.  85,  1901. 

2  Stewart,  E.,  and  Greaves,  J.  Ev  The  Production  and  Movement  of 
Nitric  Nitrogen  in  Soil;  Centbl.  f.  Bakt.,  II.  Band  34,  S.  115-147, 
1912.  '       '  '  ' 

'Brown,  B.  E.,  and  Maclntire,  W.  H.,  Seasonal  Nitrification,  Soil 
Moisture  and  Lime  Requirement  in  Four  Plats  Receiving  Sulfate  of 
Ammonia;  Penn.  Agr.  Exp.  Sta.,  Eep.  1909-1910,  pp.  57-63. 

4  Lyon,  T.  L.,  and  Bizzell,  J.  A.,  Some  Relations  of  Certain  Higher 
Plants  to  the  Formation  of  Nitrates  in  Soils;  Cornell  Agr.  Exp.  Sta., 
Memoir  1,  1913.  6  * 


SOIL  ORGANISMS 


423 


soil  management  as  well  as  the  effect  of  nitrification  on  plant 
growth. 

The  influence  of  plants  on  nitrification  is  not  confined  to 
the  period  in  which  they  are  growing  on  the  soil.  Lyon  and 
Bizzell,  in  the  investigation  previously  mentioned,  found  that 
certain  plants  grew  better  when  preceded  by  one  species 
rather  than  by  another.  These  authors,  as  already  explained, 
have  suggested  that  certain  higher  plants  directly  influence 
nitrification  with  varying  intensity.  The  question  now  arises 
as  to  the  possibility  of  such  plants  influencing  the  process  of 
nitrate  formation  after  their  removal. 

The  following  data  from  Lyon  and  Bizzell  suggest  that, 
while  the  effect  is  variable,  plants  seem  definitely  to  influence 
the  production  of  nitrates  during  the  season  after  they  have 
been  removed.    All  of  the  plats  were  kept  bare  in  1911. 


Table  XCII 


Season  1910 

Nitrates  in  Soil  Kept 
Bare  in  1911 
Parts  Per  Million 

Treatment 
in  1910 

Nitrates  in 

Soil,  Parts 

per  Million, 

Seasonal 

Average 

Nitrogen 

in  Crop, 

Pounds  Per 

Acre 

MayI 

June  28 

Maize 

167 
136 

104 
108 

90 
126 

3 
43 

29 

52 
50 

28 
43 

22 
36 

37 

Bare 

35 

Potatoes 

Bare 

26 
32 

Oats 

22 

Bare 

33 

These  results  indicate  that  maize  exerts  a  stimulating  influ- 
ence during  the  following  summer.  Oats  and  potatoes  seem 
to  depress  nitrate  accumulation. 

232.  Relation  of  nitrification  to  soil  fertility. — In  spite 
of  the  immense  amount  of  work  that  has  been  done  on  the  bio- 


424        NATURE  AND  PROPERTIES  OF  SOILS 

logical  problems  of  the  soil,  no  definite  relationships  have 
been  established  between  any  given  transformation  and  the 
productivity  of  the  soil.  General  correlations  have  been  re- 
peatedly observed x  but  specific  relationships,  when  recorded, 
are  difficult  to  ascribe  to  other  than  chance  concordance.  Of 
all  of  the  biological  transformations,  nitrification  seems  most 
likely  to  correlate  with  productivity,  since  most  plants  use 
large  amounts  of  nitrate  nitrogen. 

Available  data  seem  to  show  that  there  is  a  general  correla- 
tion between  the  nitrifying  capacity  of  soils  and  their  crop- 
producing  power.2  Such  a  statement,  however,  does  not  imply 
that  the  productivity  of  soils,  insofar  as  nitrogen  is  a  limiting 
factor,  is  especially  controlled  by  nitrification.  Arable  soils 
usually  contain  abundant  nitrifying  organisms,  which  seem 
to  oxidize  ammonia  to  the  nitrate  form  as  fast  as  it  is  pro- 
duced. It  would  appear  that  nitrification  is  only  one  of  the 
many  factors  that  govern  productivity,  a  high  nitrate  content 
of  a  soil  accompanying,  rather  than  controlling,  high  crop 
production. 

233.  Reduction  of  nitrates  and  allied  compounds. — Ni- 
trates may  be  removed  from  the  soil  in  three  ways:  (1)  by 
drainage,  (2)  by  plant  absorption,  and  (3)  by  reduction  to 
free  nitrogen.  The  loss  of  nitrogen  by  leaching  and  by  crop- 
ping has  already  been  adequately  treated.  It  has  been  shown, 
for  example  (see  par.  163),  that  as  high  a  loss  as  77  pounds  of 
nitrogen  to  the  acre  a  year  may  be  expected  from  a  heavy 

1Ashby,  S.  F.,  The  Comparative  Nitrifying  Power  of  Soils;  Jour. 
Chem.  Soc,  London,  Vol.  85,  pp.  1158-1170,  1904. 

Russell,  E.  J.,  and  Hutchinson,  H.  B.,  The  Effect  of  Partial  Sterili- 
zation of  Soils  on  the  Production  of  Plant  Food;  Jour.  Act.  Sci.,  Vol. 
Ill,  pp.  111-144,  1909.  * 

Kellerman,  K.  F.,  and  Allen,  E.  R.,  Bacteriological  Studies  of  the 
Truckee-Carson  Irrigation  Project;  U.  S.  Dept.  Act.,  Bur.  Plant  Ind., 
Bui.  211,  1911.  *   '  ' 

Brown,  P.  E.,  Relation  Between  Certain  Bacterial  Activities  in  Soils 
and  Their  Crop  Producing  Power;  Jour.  Act.  Res.,  Vol.  V,  pp.  855- 
869,  1916.  '  ^F 

aGainey,  P.  L.,  The  Significance  of  Nitrification  as  a  Factor  in  Soil 
Fertility;  Soil  Sci.,  Vol.  Ill,  No.  5,  pp.  399-416,  1917. 


SOIL  ORGANISMS  425 

soil  through  the  combined  influence  of  cropping  and  drainage. 
This  is  equivalent  to  a  removal  of  about  520  pounds  of  sodium 
nitrate  as  far  as  the  nitrogen  contained  is  concerned. 

While  the  removal  of  nitrogen  from  the  soil  is  due  very 
largely  to  the  phenomena  just  referred  to,  the  loss  of  nitro- 
gen through  reduction  demands  a  certain  amount  of  atten- 
tion. Reduction  includes  the  change  of  nitrates  to  nitrites, 
to  ammonia  and  even  to  free  nitrogen.1  In  the  same  way 
nitrites  may  be  reduced  to  ammonia  and  the  latter  to  ele- 
mental nitrogen.  When  the  process  is  carried  to  completion 
there  is  opportunity  for  an  escape  of  some  nitrogen  to  the  at- 
mospheric air.  The  loss  of  nitrogen  is  not  the  important  con- 
sideration, however.  The  interference  with  plant  nutrition, 
which  naturally  occurs,  is  much  more  serious  and  justifies 
the  attention  which  the  phenomena  have  received  from  bac- 
teriologists. 

The  number  of  organisms  that  are  capable  of  accomplishing 
one  or  more  of  the  reduction  processes  is  very  large.  This  is 
due  to  the  facultative  character  of  the  soil  flora,  which  is 
able  to  alter  its  functions  to  suit  the  conditions.  Thus  B. 
mycoides,  which  is  a  normal  decay  and  ammonifying  organ- 
ism, may,  under  anaerobic  conditions  become  a  vigorous  re- 
ducing agent.  Other  specific  reducing  organisms  are : — B 
ramosus  and  B.  pestifer,  B.  subtilis,  B.  mesenterious  vvX- 
gatus,  B.  denitrificans,  and  many  others.  It  is  probable  that 
fungi  also  are  able  to  effect  the  transformation. 

Most  of  the  reducing  bacteria  perform  their  functions  only 
in  presence  of  a  limited  amount  of  oxygen,  while  others  can 
operate  in  the  presence  of  a  more  liberal  supply.  In  general, 
thorough  aeration  of  the  soil  impedes  the  process  to  a  consid- 
erable degree.  Straw  apparently  carries  an  abundant  supply 
of  such  organisms,  and  it  is  consequently  possible  to  reach  a 

1  The  reaction  may  be  illustrated  empirically  as  follows : 
2HNOf3  —  2HN02  +  02. 
4HN02  =  2H20  +  2N2  +  302. 
HN03  +  H20  =  NH3  +  202. 


426        NATURE  AND  PROPERTIES  OF  SOILS 

point  in  manuring  at  which  reduction  takes  place.  When 
fifty  tons  or  more  of  farm  manure,  in  addition  to  a  nitrate 
fertilizer,  are  added  to  the  soil,  unfavorable  reactions  may 
occur.  Plowing  under  heavy  crops  of  green-manure  may 
produce  the  same  result.  In  either  case  the  best  way  to  over- 
come the  difficulty  is  to  allow  the  organic  matter  partly  to  de- 
compose before  adding  the  fertilizer.  The  removal  of  the 
easily  decomposable  carbohydrates  needed  by  the  reducing 
organisms  decreases  or  precludes  their  activity  in  this 
direction. 

Under  ordinary  farm  conditions  conversion  to  free  nitrogen 
is  of  no  significance  in  the  soil  where  proper  drainage  and 
good  tillage  are  practiced.  Warington  '  showed  that  if  an 
arable  soil  is  kept  saturated  with  water  to  the  exclusion  of  air, 
nitrates  added  to  the  soil  are  decomposed,  with  the  evolution 
of  nitrogen  gas.  As  lack  of  drainage  is  usually  most  pro- 
nounced in  early  spring,  when  the  soil  is  likely  to  be  depleted 
of  nitrates,  it  is  not  likely  that  much  loss  occurs  in  this  way 
unless  a  nitrate  fertilizer  has  been  added.  Among  the  many 
difficulties  arising  from  poor  drainage  the  reduction  of  an 
expensive  fertilizer  may  be  no  inconsiderable  item. 

234.  Assimilation  of  nitrates  ajid  allied  compounds.2 — 
In  addition  to  the  nitrate-reducing  organisms  already  men- 
tioned, there  are  other  bacteria  and  fungi  that  utilize  nitrates, 
nitrites,  and  ammonia.  Like  higher  plants,  they  convert 
the  nitrogen  into  organic  nitrogenous  substances.  The  proc- 
ess is  therefore,  one  of  synthesis,  rather  than  of  reduction  al- 
though reduction  often  occurs  at  the  beginning  of  the  proc- 
ess. As  such  organisms  operate  in  the  dark,  they  must  have 
organic  acids  or  carbohydrates  as  a  source  of  energy.  This 
means  of  nitrate  disappearance  is  probably  of  much  more 

1  Warington,  E.,  Investigations  at  Bothamsted  Experimental  Station; 
U.  S.  Dept.  Agr.,  Office  of  Exp.  Sta.,  Bui.  8,  p.  64,  1892. 

2  The  term  denitrification  is  often  used  in  referring  to  the  reduction 
and  assimilation  of  nitrates  and  allied  compounds  in  the  soil.  The  word 
is  so  loosely  used  in  soil  literature  that  it  has  seemed  best  to  ignore  it, 
at  least  for  the  present. 


SOIL  ORGANISMS 


427 


practical  importance  than  nitrate  reduction,  yet  even  less  is 
known  regarding  the  phenomena.  Many  different  forms  of 
bacteria  and  fungi  are  probably  capable  of  assimilating 
nitrogen,  but  what  conditions  favor  their  activity  in  this  re- 
spect cannot  be  stated  definitely.1  To  make  the  problem  more 
intricate  higher  plants  seem  to  be  a  factor  to  a  certain  ex- 
tent in  this  type  of  nitrate  disappearance.  Seasonal  influ- 
ences also  have  been  noted,  which  suggest  the  possibility  of  a 
special  nitrate  assimilating  flora. 

Nitrate  accumulation  always  proceeds  slowly  on  sod  land, 
especially  if  the  soil  is  heavy.  Lack  of  sufficient  moisture  or 
unfavorable  temperature  relations  do  not  always  adequately 
account  for  this  phenomenon.  An  experiment  at  Cornell  Uni- 
versity 2  is  typical  of  the  conditions  mentioned  above.  In  this 
case,  maize  and  grass  were  grown  side  by  side,  the  nitrates 
being  determined  at  frequent  intervals  during  the  season. 
The  nitrates  are  expressed  in  parts  per  million  of  dry  soil  for 
the  various  months. 

Table  XCIII 

NITRATES    IN    PARTS   PER    MILLION    UNDER    MAIZE    AND    SOD. 
CORNELL  UNIVERSITY. 


Nitrates,  Parts  per  Million  Dry 


Month 


1  Murray  has  found  at  the  Washington  Agricultural  Experiment  Sta- 
tion that  the  addition  of  straw  to  the  soil  markedly  aided  the  bacterial 
utilization  of  nitrates.  The  numbers  of  bacteria  increased  without 
reference  to  the  groups  present. 

Murray,  T.  J.,  The  Effect  of  Straw  on  the  Biological  Soil  Processes; 
Soil  Sci.,  Vol.  XII,  No.  3,  pp.  233-259,  1921. 

*  Unpublished  data.     Cornell  Agr.  Exp.  Sta.,  Ithaca,  N.  Y. 


428        NATURE  AND  PROPERTIES  OF  SOILS 

The  high  nitrate  accumulation  under  the  maize  is  probably 
due  to  the  tillage  and  aeration  which  the  soil  received  and 
possibly  to  the  direct  stimulation  of  the  crop  on  nitrification. 
The  low  amounts  of  nitrate  nitrogen  in  the  grass  land  are 
probably  due,  at  least  partially,  to  the  influence  of  the  sod  in 
encouraging  nitrate  assimilation  by  the  soil  organisms.  The 
nitrifying  power  of  the  sod  soil  is  probably  much  greater 
than  the  data  just  presented  would  lead  one  to  suspect.  Sodi- 
um nitrate  applied  to  grass  at  Cornell  University  was  found 
to  be  changed  to  other  than  the  nitrate  form  very  rapidly, 
even  when  the  amounts  added  were  extremely  large.  This 
rapid  disappearance  of  the  nitrate  form  of  nitrogen  is  not 
readily  accounted  for  by  cropping  and  drainage  removal. 
Such  facts  lend  considerable  plausibility  to  the  suggestions 
made  above,  regarding  the  encouragement  which  the  synthetic 
removal,  especially  of  nitrates,  receives  from  organisms  when 
the  soil  is  under  a  grass  crop. 

The  synthetic  removal  of  nitrates,  nitrites,  and  ammonia 
assumes  considerable  importance  at  certain  times  of  the  year. 
It  seems  to  be  a  natural  means  of  conserving  an  important 
soil  constituent,  since  nitrate  nitrogen  is  extremely  soluble 
and  easily  lost  by  drainage.  ,The  nitrogen  thus  affected  is 
changed  to  a  more  or  less  stable  form,  from  which  nitrates 
may  be  produced  during  the  following  year.  The  use  of  a 
cover-crop  in  an  orchard  during  the  late  summer  and  fall  is 
often  practiced.  A  disappearance  of  nitrates  but  not  a  loss  of 
nitrogen  thus  occurs  and  the  trees  are  early  forced  into  the 
resting  stage. 

235.  Natural  acquisition  of  nitrogen  by  the  soil. — Since 
all  of  the  nitrogen  now  found  in  the  soil  was  probably  ac- 
quired from  the  atmosphere,  the  natural  forces  which  facili- 
tate such  a  transfer  assume  considerable  practical  importance. 
The  more  rapid  the  natural  acquisition  of  nitrogen  from  the 
air,  the  less  serious  will  be  the  nitrogen  problem  in  agricul- 
tural practice. 


SOIL  ORGANISMS 


429 


Three  modes  of  nitrogen  fixation  are  usually  recognized :  (1) 
rain-water  additions,  (2)  the  action  of  soil  organisms  func- 
tioning independently  of  living  higher  plants,  and  (3)  the 
influence  of  organisms  functioning  parasitically  or  symbi- 
otically  in  the  soil. 

236.  Additions  of  nitrogen  in  rainwater. — Nitrogen  oc- 
curring in  rainwater  is  generally  in  the  nitrate  and  ammoni- 
cal  forms  and,  consequently,  is  readily  available  to  plants. 
The  amounts  thus  brought  down  are  quite  variable,  usually 
fluctuating  markedly  with  season  and  location.  The  follow- 
ing table  gives  some  of  the  more  important  findings  regarding 
the  amount  of  nitrogen  thus  added  to  the  soil  in  various  parts 
of  the  world. 


Table  XCIV 


Years 

OF 

Record 

Rainfall 

IN 

Inches 

Pounds    to   the 
Acre  a  Year 

Location 

Ammo- 

NIACAL 

Nitrogen 

Nitrate 
Nitrogen 

Harpenden,   England 1 .  .  .  . 

Garf ord,  England  2 

Flahult,  Sweden  3 

28 
3 
1 

2 

10 

6 

28.8 
26.9 
32.5 
27.6 

23.4 
29.3 

2.64 
6.43 
3.32 
4.54 

4.02 

4.42 

11.50 

1.33 
1.93 
1.30 

Groningen,  Holland  * 

Bloemfontein  and  Durban, 
S.   Africa  5 

1.46 
1.39 

Ottawa,  Canada  6 

2.16 

Ithaca,  New  York  7 

1.01 

1  Russell,  E.  J.,  and  Richards,  E.  H.,  The  Amount  and  Composition 
of  Bain  Falling  at  Bothamsted;  Jour.  Agr.  Sci.,  Vol.  IX,  pp.  309-337, 
1919. 

aCrowther,  C,  and  Ruston,  A.  G.,  The  Nature,  Distribution  and 
Effects  upon  Vegetation  of  Atmospheric  Impurities  In  and  Near  an 
Industrial  Town;  Jour.  Agr.  Sci.,  Vol.  IV,  pp.  25-55,  1911. 

3  Von  Feilitzen,  H.,  and  Lugner,  I.,  On  the  Quantity  of  Ammonia 
and  Nitric  Acid  in  Bainwater  Collected  Near  Flahult,  in  Sweden;  Jour. 
Agr.  Sci.,  Vol.  Ill,  pp.  311-313,  1910. 

4Hudig,  J.,  The  Amounts  of  Nitrogen  as  Ammonia  and  Nitric  Add 


430        NATURE  AND  PROPERTIES  OF  SOILS 

Apparently  the  ammoniacal  nitrogen  is  always  consider- 
ably larger  in  amount  than  that  in  the  nitrate  form.  It  is 
also  noticeable  that  while  the  nitrate  nitrogen  is  about  the 
same  for  every  station,  the  nitrogen  in  the  form  of  ammonia 
shows  wide  variations.  The  quantities  at  Ithaca,  New  York, 
are  considerably  larger  than  those  from  any  other  station. 
Considering  the  figures  as  a  whole,  it  seems  fair  to  assume 
that  on  the  average  about  4y2  pounds  of  ammoniacal  and  iy2 
pounds  of  nitrate  nitrogen  fall  on  every  acre  of  soil  yearly 
in  rainwater.  Assuming  that  all  of  this  nitrogen  passes  into 
the  soil,  an  average  gain  to  the  acre  of  6  pounds  of  nitrogen 
may  be  expected. 

It  is  interesting  at  this  point  to  compare  such  a  gain  with 
the  annual  loss  of  nitrogen  from  the  soil.  The  removal  of 
nitrogen  from  the  Cornell  lysimeter  soils  (see  par.  163), 
through  drainage  and  cropping  combined,  amounted  to  69.0, 
77.8  and  56.9  pounds  yearly  to  the  acre,  respectively,  for  a 
bare  soil,  one  carrying  a  standard  rotation,  and  one  continu- 
ously in  grass.  While  a  gain  of  6  pounds  to  the  acre  yearly 
seems  rather  insignificant  in  comparison  to  these  figures,  such 
an  addition  is  of  considerable  importance  over  a  period  of 
years,  and  has  had  much  to  do  with  the  accumulation  of  the 
nitrogen  of  our  arable  soils.  Such  a  gain  is  equivalent  in  a 
practical  way  to  the  addition  of  about  40  pounds  of  commer- 
cial sodium  nitrate  to  the  acre  yearly. 

237.  Acquisition  of  nitrogen  by  free-fixing  organisms. — 
While  it  has  long  been  known  that  the  soil  contains  a  great 
variety  of  organisms,  it  is  only  in  recent  years  that  it  has  been 

in  the  'Rainwater  Collected  at  Uithuizer-Meeden,  Gronigen;  Jour.  Agr. 
Sci.,  Vol.  IV,  pp.  260-269,  1912. 

5  Juritz,  C.  P.,  Chemical  Composition  of  Rain  in  the  Union  of  South 
Africa;  S.  Africa  Jour.  Sci.,  Vol.  10,  pp.  170-193,  1914. 

"Shutt,  F.  T.,  and  Dorrance,  R,,  The  Nitrogen  Compounds  of  Rain 
and  Snow;  Proc.  and  Trans.  Roy.  Soe.  Canada,  Vol.  XI,  No.  3,  pp.  63-71, 

T  Wilson,  B.  D.,  Nitrogen  in  the  Rainwater  at  Ithaca,  New  York; 
Soil  Sci.,  Vol.  XI,  No.  2,  pp.  101-110,  1921. 


SOIL  ORGANISMS  431 

definitely  shown  that  certain  of  these  organisms  have  the 
power  of  utilizing  atmospheric  nitrogen,  which  later  becomes 
a  part  of  the  nitrogenous  matter  of  the  soil.  Boussingault  * 
in  1858  suggested  the  possibility  of  such  a  phenomenon,  but 
it  was  not  until  1883  that  Berthelot 2  began  experiments  by 
which  he  demonstrated  that  bare  soils  appreciably  increase 
in  nitrogen  on  exposure  under  such  conditions.  Winograd- 
ski 3  in  1894  was  the  first,  however,  to  isolate  an  organism 
capable  of  affecting  such  a  transformation.  This  bacterium 
was  an  anaerobic,  rod-shaped  organism  producing  spores 
and  a  boat-shaped  mass  (Clostridium)  ;  hence  the  name,  Clos- 
tridium pastorianum.     It  is  very  widely  distributed  in  soils. 

The  most  important  organism  fixing  nitrogen  independently 
in  the  soil  was  discovered  by  Beijerinck  4  in  1901.  This  or- 
ganism was  an  aerobic  bacillus  to  which  he  gave  the  name 
Azotobacter.  It  was  at  first  thought  that  this  bacillus  could 
not  fix  nitrogen  unless  certain  other  organisms,  such  as  Granu- 
lobacter,  Radiobacter  and  Aerobacter,  were  also  present.  Lip- 
man  5  has  shown  this  idea  to  be  erroneous,  although  the  effi- 
ciency of  Azotobacter  is  much  higher  in  mixed  than  in  pure 
cultures.  A  number  of  different  species  of  Azotobacter  have 
been  studied,  the  A.  chroococcum  apparently  being  the  most 
widespread. 

Clostridium  pastorianum  and  Azotobacter  are  by  no  means 
the  only  soil  organisms  capable  of  fixing  nitrogen.     Among 

1  See  Voorhees,  E.  B.,  and  Lipman,  J.  G.,  A  Review  of  Investigations 
in  Soil  Bacteriology ;  U.  S.  Dept.  Agr.,  Office  of  Exp.  Sta.,  Bui.  194, 
1907. 

2  Berthelot,  M.,  Becherches  nouvelles  sur  les  microorganisms  fixateurs 
de  l' azote;  Compt.  Rend.  Acad.  Sci.  Paris,  Tome  115,  pp.  569-574  and 
842-849,   1892-93. 

3  Winogradsky,  S.,  Sur  V assimilation  de  V azote  gazeux  de  I' atmosphere 
par  les  microbes;  Compt.  Rend.  Acad.  Sci.  Paris,  Tome  118,  pp.  353-355, 
1894. 

4Beijerinck,  M.  W.,  Tiber  Oligonitrophile  Mikroben;  Centrbl.  Bakt., 
II,  Bd.  7,  S.  561-582,  1901. 

5Lipman,  J.  G.,  Experiments  on  the  Transformation  and  Fixation  of 
Nitrogen  by  Bacteria;  N.  J.  Agr.  Exp.  Sta.,  24th  Ann.  Rep.,  pp.  217-285, 
1903. 


432        NATURE  AND  PROPERTIES  OF  SOILS 

bacteria,  B.  mesentericus,  B.  pneumonia,  B.  radiobacter,  B. 
amylobacter,  B.  prodigiosus,  B.  asterosporus,  and  B.  lactis 
viscusus  have  certain  capacities  in  this  direction.  Duggar 
and  Davis x  have  shown  that  certain  filamentous  fungi,  such 
as  Phoma  beta,  Aspergillus  niger,  Pencillium  digitatum, 
and  others  have  the  ability  of  utilizing  atmospheric  nitrogen. 
The  power  of  fixing  nitrogen  is,  therefore,  possessed  by  a 
large  number  of  different  organisms,  yet  from  the  data  now 
at  hand  the  Azotobacter  group  seems  to  be  of  the  greatest 
economic  importance.  The  nitrogen  fixed  enters  the  nitrogen 
cycle  when  the  organisms  die,  undergoing  decay,  ammonifica- 
tion  and  nitrification,  thus  becoming  available  to  higher  plants. 

238.  Conditions  for  azofication  and  the  amount  of  nitro- 
gen fixed.2 — The  term  azofication  relates  to  the  fixation  of 
nitrogen  by  the  Azotobacter  group,  although  it  may  be  used 
loosely  in  reference  to  all  free-fixing  activities.  The  soil  con- 
ditions favorable  to  this  phenomenon  are  those  which  are  opti- 
mum for  higher  plants.  This  is  especially  true  regarding 
aeration,  temperature,  and  moisture  relations.  The  process 
is  encouraged  by  the  application  of  lime  when  soils  are  acid 
and  seem  to  require  considerable  phosphorus.  This  element  is 
probably  utilized  in  building  up  proteins  within  the  bodies 
of  the  organisms.  Potassium,  sulfur,  iron,  and  magnesium 
seem  also  to  be  essential  to  the  phenomenon.  The  Azotobac- 
ter themselves  are  influenced  by  catalytic  agents  such  as 
manganese. 

Since  considerable  energy  is  required  for  nitrogen  fixation 
the  presence  of  organic  matter  in  the  soil  becomes  very  im- 
portant in  this  regard.  Almost  any  non-toxic  organic  ma- 
terial may  serve  as  a  source  of  energy,  even  cellulose  being 
very  effective.     Farm  manure  seems  especially  to  encourage 

Nuggar,  B.  M.,  and  Davis,  A.  R.,  Studies  in  the  Physiology  of  the 
Fungi;  Ann.  Mo.  Bot.  Garden,  Vol.  33,  pp.  413-437,  1916. 

2 A  very  excellent  review  of  literature  and  discussion  of  Azofication: 
Greaves,  J.  E.,  Azofication;  Soil  Sci.,  Vol.  VI,  No.  3,  pp.  163-217, 
1918.  '    FF 


SOIL  ORGANISMS  433 

nitrogen  fixation.  The  maintenance  of  a  fair  supply  of  soil 
organic  matter  is,  therefore,  as  important  as  the  regulation 
of  the  temperature,  the  oxygen,  the  moisture,  and  the  reac- 
tion of  the  soil.  While  the  presence  of  nitrates  in  small 
amounts  seems  to  stimulate  azofication,  large  quantities  of 
nitric  nitrogen  tend  to  lessen  nitrogen  fixation. 

The  amount  of  nitrogen  fixed  in  the  soil  by  organisms  func- 
tioning independently  of  higher  plants  is,  as  might  be  ex- 
pected, a  variable  quantity.  Hall *  considers  it  to  be  on  the 
average  about  25  pounds  yearly  to  the  acre,  Greaves 2  25 
pounds,  Lbhnis 3  36  pounds,  and  Lipman 4  from  15  to  40 
pounds.  As  a  basis  for  calculation  25  pounds  is  perhaps  a 
conservative  and  reasonable  figure.  A  comparison  of  this 
figure  with  the  6  pounds  of  nitrogen  brought  down  yearly 
in  rain-water,  indicates  that  the  free-fixing  organisms  are 
four  or  &ve  times  more  important  than  rainfall  as  a  source  of 
nitrogen. 

239.  Bacillus  radicicola  and  its  relationship  to  the  host 
plant. — It  has  long  been  recognized  by  farmers  that  certain 
crops,  as  clover,  alfalfa,  peas,  beans,  and  some  others,  im- 
prove the  soil,  making  it  possible  to  grow  larger  crops  of 
cereals  after  these  plants  have  occupied  the  land.  Within 
the  last  century  the  benefit  has  been  traced  to  the  fixation 
of  nitrogen  through  the  agency  of  bacteria  contained  in  nod- 
ules on  the  roots.  The  specific  plants  so  affecting  the  soil 
were  found  to  be,  with  a  few  exceptions,  those  belonging  to 
the  family  of  legumes.  It  has  furthermore  been  demonstrated 
that  the  host  plant  is  generally  able  to  appropriate  some  of 
the  nitrogen  so  fixed  and  thus  benefit  by  the  relationship. 
The  phenomenon  was  fully  explained  in  1886  by  Hellreigel 

1  Hall,  A.  D.,  On  the  Accumulation  of  Fertility  by  Land  Allowed  to 
Bun  Wild;  Jour.  Agr.  Sci.,  Vol.  I,  pp.  241-249,  1905. 

3  Greaves,  J.  E.,  Azofication;  Soil  Sci.,  Vol.  VI,  No.  3,  pp.  163-217, 
1918. 

3  Lohnis,  F.,  and  Westermann,  F.,  Tiber  SticTcstojf  fixierende  Bakterien, 
IV.    Centrbl.  f.  Bakt.,  II,  Bd.  22,  S.  234-254,  1909. 

4 Lipman,  J.  G.,  Marshall's  Microbiology,  p.  343,  1917. 


434        NATURE  AND  PROPERTIES  OF  SOILS 

and  Wilfarth.  The  organisms,  of  which  there  are  a  number 
of  strains,  are  called  Bacillus  radicicola. 

The  organisms  living  in  the  root  nodules  take  free  nitrogen 
from  the  air  in  the  soil,  and  the  host  plant  secures  it  in  some 
form  from  the  bacteria  or  their  products.  The  presence  of  a 
certain  species  of  bacteria  is  necessary  for  the  formation  of 
tubercles.  Leguminous  plants  grown  in  cultures  or  in  soil 
not  containing  the  necessary  bacteria  do  not  form  nodules 
and  do  not  utilize  atmospheric  nitrogen,  the  result  being  that 
the  crop  produced  is  less  in  amount  and  the  percentage  of 
nitrogen  in  the  crop  is  lower  than  if  nodules  were  formed. 

The  nodules  are  not  normally  a  part  of  leguminous  plants, 
but  are  evidently  caused  by  an  irritation  of  the  root  sur- 
face, much  as  a  gall  is  caused  to  develop  on  a  leaf  or  a  branch 
of  a  tree  by  an  insect.  In  a  culture  containing  the  proper 
bacteria  the  prick  of  a  needle  on  the  root  surface  will  cause  a 
nodule  to  form  in  the  course  of  a  few  days.  The  entrance  of 
the  organism  is  effected  through  a  root-hair  which  it  pene- 
trates, and  it  may  be  seen  as  a  filament  extending  the  entire 
length  of  the  hair  and  into  the  cortex  cells  of  the  root,  where 
the  growth  of  the  tubercle  starts. 

Even  where  the  causative  bacteria  occur  in  cultures  or  in 
the  soil,  a  leguminous  plant  may  not  secure  any  atmospheric 
nitrogen,  or  perhaps  only  a  small  quantity,  if  there  is  an 
abundant  supply  of  readily  available  combined  nitrogen  on 
which  the  plant  may  draw.  The  bacteria  have  the  ability  to 
utilize  combined  as  well  as  uncombined  nitrogen,  and  prefer 
to  have  it  in  the  former  condition.  On  soils  rich  in  nitrogen, 
legumes  may,  therefore,  add  little  or  no  nitrogen  to  the  soil, 
if  the  above  ground  portion  of  the  crop  is  not  plowed  under; 
while  in  properly  inoculated  soils  deficient  in  nitrogen  an 
important  gain  of  nitrogen  may  result. 

While  B.  radicicola  is  considered  the  organism  common  to 
all  leguminous  plants,  it  is  now  known  that  the  organisms 
from  one  species  of  legume  are  not  equally  well  adapted  to 


SOIL  ORGANISMS  435 

the  production  of  tubercles  on  other  leguminous  species.  Cer- 
tain cross  inoculations  are,  however,  very  successful.  The 
organisms  seem  to  be  interchangeable  within  the  clovers,  the 
vetches  and  the  bean  family.  The  organisms  from  sweet 
clover  and  burr  clover  will  inoculate  alfalfa,  while  the  bac- 
teria may  be  transferred  from  vetch  to  field  pea  or  from  cow- 
pea  to  velvet  bean. 

It  has  been  shown  by  several  investigators  that  bacteria 
from  the  nodules  of  legumes  are  able  to  fix  atmospheric  nitro- 
gen even  when  not  associated  with  leguminous  plants.  There 
would  seem  to  be  no  doubt,  therefore,  that  the  fixation  of 
nitrogen  in  the  tubercles  of  legumes  is  accomplished  directly 
by  this  organism,  not  by  the  plant  itself  nor  through  any  com- 
bination of  the  plant  and  the  organism.  The  relationship  is, 
therefore,  parasitical  rather  than  strictly  symbiotic,  although 
the  host  plant  benefits  from  the  relation.  The  part  played 
by  the  plant  is  doubtless  to  furnish  the  carbohydrates  which 
are  required  in  considerable  quantities  by  all  nitrogen-fixing 
organisms  and  which  the  legumes  are  able  to  supply  in  large 
amounts.  The  utilization  of  large  quantities  of  carbohydrates 
by  the  nitrogen-fixing  bacteria  in  the  tubercles  may  also  ac- 
count for  the  small  proportion  of  non-nitrogenous  organic 
matter  in  the  plants. 

How  the  plant  absorbs  this  nitrogen  after  it  has  been 
secured  by  the  bacteria  is  not  well  understood  nor  is  it  known 
in  exactly  what  form  the  nitrogen  is  at  first  fixed,  although 
amino  and  amide  nitrogen  very  soon  appear.1  Early  in  the 
growth  of  the  tubercle,  a  mucilaginous  substance  is  produced, 
which  permeates  the  tissues  of  the  plant  in  the  form  of  long 
slender  threads  containing  the  bacteria.  These  threads  de- 
velop by  branching  or  budding,  and  form  what  have  been 
called  Y  and  T  forms,  known  as  bacteroids,  which  are  peculiar 
to  these  bacteria.     The  threads  finally  disappear,  and  the 

1Strowd,  W.  H.,  The  Forms  of  Nitrogen  in  Soybean  Nodules;  Soil 
Sci.,  Vol.  XI,  No.  2,  pp.  123-130,  1921. 


436        NATURE  AND  PROPERTIES  OF  SOILS 

bacteria  diffuse  themselves  more  or  less  through  the  tissues  of 
the  root.  What  part  the  bacteroids  play  in  the  transfer  of 
nitrogen  is  not  known.  It  has  been  suggested  that  in  this 
form  the  nitrogen  is  absorbed  by  the  tissues  of  the  plant.  It 
seems  quite  likely  that  the  nitrogen  compounds  produced 
within  the  bacterial  cells  are  diffused  through  the  cell-wall  and 
absorbed  by  the  plant. 

240.  The  practical  importance  of  B.  radicicola. — The 
nitrogen  fixed  by  the  nodule  organisms  may  go  in  three  di- 
rections in  the  soil.  It  may  be  absorbed  by  the  host  plant, 
the  latter  benefiting  greatly  by  the  association.  This  rela- 
tionship has  already  been  discussed.  Secondly,  the  nitrogen 
may  pass  in  some  way  into  the  soil  itself  and  benefit  a  crop 
associated  with  the  legume.  Thirdly,  the  nodules  may  decay, 
when  the  legume  dies  or  is  turned  under,  the  nitrogen  be- 
coming available  to  the  succeeding  crop. 

The  relationship  between  associated  legumes  and  non- 
legumes  has  been  particularly  studied  by  Lyon  and  Bizzell 1 
and  by  Lipman.2  It  has  been  quite  definitely  proven  that  the 
non-legume  may  be  greatly  benefited  by  the  association  under 
some  conditions.  This  accounts  for  the  practice  of  growing 
timothy  with  clover,  which  has  been  common  for  centuries. 
Just  how  the  transfer  of  nitrogen  is  facilitated  yet  remains 
to  be  shown. 

The  beneficial  influences  of  such  legumes  as  clover,  vetch, 
and  alfalfa  on  the  succeeding  crops  has  long  been  taken  ad- 
vantage of  in  practical  agriculture.  Until  recently  the  stimu- 
lation has  been  ascribed  to  an  actual  increase  of  nitrogen  in 
the  soil,  due  to  the  growth  of  the  legume  and  the  activity  of 
its  nodule  organisms.  This  will  not  always  account  for  the 
phenomenon,  since  it  has  been  shown  by  a  number  of  investi- 

1Lyon,  T.  L.,  and  Bizzell,  J.  A.,  Availability  of  Soil  Nitrogen  in 
Relation  to  the  Basicity  of  the  Soil  and  to  the  Growth  of  Legumes; 
■  Jo2ur\  Ind-  and  Eng-  chem.,  Vol.  2,  No.  7,  pp.  313-315,  1910. 

2  Ldpman,  J.  G.,  The  Associative  Growth  of  Legumes  and  Non-Legumes. 
N.  J.  Agr.  Exp.  Sta.,  Bui.  253,  1912. 


SOIL  ORGANISMS  437 

gators  that  the  continuous  growing  of  legumes,  the  tops  being 
removed  as  forage,  does  not  always  increase  the  nitrogen  con- 
tent of  the  soil  to  any  greater  extent  than  does  a  non-legumi- 
nous crop. 

The  results  of  Swanson  *  are  particularly  striking  in  this 
respect.  This  investigator  sampled  a  number  of  fields  in 
Kansas  that  had  grown  alfalfa  continuously  for  twenty  or 
thirty  years,  at  the  same  time  obtaining  soil  from  contiguous 
native  sod.  In  most  cases  the  alfalfa  soil  was  lower  in 
nitrogen  than  the  sod.  Lyon  and  Bizzell 2  found  practically 
the  same  content  of  nitrogen  in  contiguous  alfalfa  and 
timothy  soils  after  the  crops  had  been  growing  six  years. 
The  maize  crop  following  the  alfalfa  was  nevertheless  much 
greater  than  that  after  the  timothy.  Since  the  soil  on  which 
a  legume  has  been  growing  generally  has  a  rather  high  nitrify- 
ing capacity,3  the  explanation  seems  to  lie  in  the  ready  avail- 
ability of  the  nitrogen  in  the  soil  which  bore  the  legume, 
rather  than  to  the  presence  of  an  especially  large  amount. 

The  amount  of  nitrogen  fixed  by  the  nodule  organisms  of 
a  leguminous  crop  is  very  uncertain.  If  the  soil  is  acid,  if 
it  contains  alkali  salts  above  a  certain  amount,  or  if  nitrates 
develop  rapidly,  nitrogen  fixation  is  markedly  retarded.  Much 
also  depends  on  the  virulence  of  the  organisms,  the  character 
of  the  legume,  the  presence  of  organic  matter,  and  other  im- 
portant conditions.    Hopkins  4  estimates  that  about  one-third 

1  Swanson,  C.  O.,  The  Effect  of  Prolonged  Growing  of  Alfalfa  on 
the  Nitrogen  Content  of  the  Soil;  Jour.  Amer.  Soc.  Agron.,  Vol.  9, 
No.  7,  pjv  305-314,  1917. 

Swanson,  C.  O.,  and  Latshaw,  W.  L.,  Effect  of  Alfalfa  on  the  Fer- 
tility Elements  of  the  Soil  in -Comparison  with  Grain  Crops;  Soil  Sci., 
Vol.  VIII,  No.  1,  pp.  1-39,  1919. 

2  Lyon,  T.  L.,  and  Bizzell,  J.  A.,  Experiments  Concerning  the  Top- 
dressing  of  Timothy  and  Alfalfa;  Cornell  Agr.  Exp.  Sta.,  Bui.  339, 
pp.  136-139,  1913. 

3  Lyon,  T.  L.,  Bizzell,  J.  A.,  and  Wilson,  B.  D.,  The  Formation  of 
Nitrates  in  a  Soil  Following  the  Growth  of  Bed  Clover  and  of  Timothy; 
Soil  Sci.,  Vol.  IX,  No.  1,  pp.  53-64,  1920. 

4  Hopkins,  C.  G.,  Soil  Fertility  and  Permanent  Agriculture,  p.  223, 
Boston,  1910. 


438        NATURE  AND  PROPERTIES  OF  SOILS 

of  the  nitrogen  of  a  normal  inoculated  legume  comes  from 
the  soil  and  two-thirds  from  the  air.  He  also  assumes  that 
one-third  of  the  nitrogen  of  the  plant  exists  in  the  roots.  Al- 
though both  of  these  assumptions  are  questionable,  they  sug- 
gest the  reason  why  the  removal  of  the  tops  of  legumes  as 
forage  allows  no  accumulation  of  nitrogen  in  the  soil. 

According  to  Hopkins,  the  nitrogen  in  the  tops  of  legumes 
is  a  rough  measure  in  general  of  the  nitrogen  fixed.  On  such 
an  assumption,  the  growth  of  red  clover  should  facilitate  the 
fixation  of  about  40  pounds  of  nitrogen  for  every  ton  of  air- 
dry  material.  On  the  same  basis,  the  figure  should  be  about 
50  pounds  for  alfalfa,  43  pounds  for  cowpeas,  and  53  pounds 
for  soybeans.  These  figures,  even  though  they  are  obviously 
incorrect,  give  some  idea  of  the  importance  of  B.  radicicola  in 
nitrogen  fixation.  The  growth  of  an  average  leguminous 
crop  under  proper  conditions  probably  is  accompanied  by  a 
fixation  of  80  to  100  pounds  of  nitrogen.-  Of  the  three  nat- 
ural methods  by  which  atmospheric  nitrogen  may  be  fixed 
by  the  soil  that  facilitated  by  the  nodule  organisms  seems 
at  first  thought  to  be  considerably  the  most  important.  It 
must  be  remembered,  however,  that  with  an  average  rotation 
a  legume  occupies  the  land  but  one  or  two  years  in  three  to 
six.  Moreover,  the  gain  of  nitrogen  in  a  fertile  soil  is  but 
slight  unless  the  crop  is  turned  under  as  a  green-manure. 
Unless  so  used  the  chief  advantages  of  growing  a  legumi- 
nous crop  lie  in  the  increase  of  soil  organic  matter,  the 
ready  and  favorable  decay  of  the  roots  and  stubble,  and  the 
opportunity  of  growing  a  high  protein  crop  without  ma- 
terially depleting  the  soil  nitrogen. 

241.  Soil  inoculation  for  legumes. — Although  the  inocu- 
lation of  the  soil  with  free-fixing  organisms  has  not  proven 
of  value,  since  such  organisms  are  always  present  and  suffi- 
ciently active  if  soil  conditions  are  favorable,  the  inoculation 
with  nodule  bacteria  is  of  considerable  practical  importance. 
Such  organism  may  never  have  been  present  in  a  soil  or  may 


SOIL  ORGANISMS  439 

have  disappeared  because  of  unfavorable  conditions.  If  leg- 
umes, especially  of  certain  types,  are  to  be  grown  most  suc- 
cessfully, the  specific  strains  of  B.  radicicola  for  that  crop 
must  be  present. 

Two  general  methods  of  inoculation  are  available:  (1)  the 
use  of  soil  from  fields  where  the  particular  legume  in  ques- 
tion is  growing  or  has  grown  successfully.;  and  (2)  the  utiliz- 
ation of  artificial  cultures  of  some  form.  Bacillus  radicicola 
is  found  in  the  soil  as  well  as  in  the  plant  nodules.  As  a 
matter  of  fact,  this  bacterium  will  live  in  the  soil  for  long 
periods,  even  if  the  host  plant  is  not  grown.  Whether  it  fixes 
nitrogen  to  any  extent  under  such  conditions  is  a  question. 
At  least  the  organism  does  not  lose  its  virulence.  Such  soil 
may  be  spread  on  the  land  to  be  inoculated  at  the  rate  of 
300  to  500  pounds  to  the  acre.  It  should  be  applied  in  the 
evening  or  on  a  cloudy  day  and  harrowed  in  as  soon  as  pos- 
sible, as  the  organisms  are  injured  by  direct  sunlight. 

The  soil  carrying  the  organism  may  also  be  mixed  after 
air-drying  with  the  seed,  the  latter  having  been  moistened  with 
a  dilute  glue  solution.1  Enough  of  the  dry  earth  sticks  to 
the  seed  to  carry  the  organisms  into  the  soil.  The  advantage 
of  this  method  is  that  the  bacteria  are  in  contact  with  the  seed 
and  the  plants  become  infected  very  soon  after  the  seeds 
germinate.  The  main  objection  to  the  soil  method  of  inocu- 
lation lies  in  the  possibility  of  spreading  plant  diseases  and 
undesirable  weeds. 

1  Dissolve  ordinary  furniture  glue  in  boiling  water,  two  handfuls  of 
glue  to  every  gallon  of  water  used,  and  allow  the  solution  to  cool.  Put 
the  seed  in  a  wash-tub,  and  then  sprinkle  enough  of  the  solution  on  the 
seed  to  moisten  but  not  to  wet  it  (one  quart  to  a  bushel  is  sufficient), 
and  stir  the  mixture  thoroughly  until  all  the  seeds  are  moistened. 

Dry  the  inoculating  soil  in  the  shade,  preferably  in  the  barn  or  base- 
ment, and  pulverize  it  thoroughly  into  a  dust.  Scatter  this  dust  over  the 
moistened  seed,  using  from  one-half  to  one  gallon  of  dirt  for  each 
bushel  of  seed,  mixing  thoroughly  until  the  seed  no  longer  stick  together. 
The  seed  is  then  ready  to  sow. 

See  Vrooman,  C,  Grain  Farming  in  the  Corn  Belt  with  Live  Stock  as 
Side  Line;  Farmers'  Bui.,  No.  704,  1916. 


440        NATURE  AND  PROPERTIES  OF  SOILS 

Within  recent  years  a  number  of  cultures  for  soil  inocula- 
tion have  been  offered  to  the  public.  The  first  of  these  util- 
ized absorbent  cotton  to  transmit  the  bacteria  in  a  dry  state 
from  the  pure  culture  in  the  laboratory  to  the  user  of  the  cul- 
ture, who  was  to  prepare  therefrom  another  culture  to  be  used 
for  inoculating  the  soil.  Careful  investigation  of  this  method 
showed  that  its  weakness  lay  in  drying  the  cultures  on  the  ab- 
sorbent cotton,  which  frequently  resulted  in  the  death  of  the 
organisms.  More  recently  liquid  cultures  have  been  placed 
on  the  market  in  this  country,  and  these  have,  in  the  main, 
proved  to  be  more  successful,  notably  those  sent  out  by  the 
United  States  Department  of  Agriculture. 

Another  very  successful  culture  medium,  now  being  used 
by  the  Department  of  Plant  Physiology  at  Cornel  University, 
is  steamed  soil.  A  soil,  favorable  to  the  development  of 
nodule  organisms  and  usually  a  sandy  loam,  is  sterilized  by 
steaming.  It  is  then  brought  up  to  optimum  moisture  and 
later  inoculated  with  a  number  of  different  strains  of  B. 
radicicola.  After  incubation  for  several  days  at  a  favorable 
temperature,  the  soil  cultures  are  ready  for  distribution.  The 
soil  is  sent  out  in  small  air-tight  cans  by  parcel  post.  The 
advantage  of  such  a  culture  is  that  the  organisms  are  viru- 
lent and  there  is  no  danger  from  plant  diseases  or  undesir- 
able weeds. 

When  a  culture  of  this  sort  is  received  it  may  be  used  in  a 
number  of  different  ways.  It  may  be  mixed  with  field  soil 
at  the  rate  of  1  pound  to  300  of  the  latter.  This  300  pounds 
of  inoculated  soil  may  then  be  spread  on  a  acre  of  land  in 
the  usual  way.  The  culture  may  also  be  disposed  of  by  the 
glue  method  or  it  may  be  suspended  in  water  and  the  extract 
sprinkled  on  the  seed  and  dried  in  the  shade.  In  either  case, 
the  seed  should  be  sown  as  soon  as  possible. 

242.  Resume. — The  biological  phases  of  the  soil  are  so  nu- 
merous and  far-reaching  that  it  is  obviously  impossible  in 
summarizing  their  practical  relationships  to  do  more  than  call 


SOIL  ORGANISM'S  441 

attention  to  certain  significant  facts.  In  the  first  place,  the 
soil  fauna  and  flora,  especially  the  latter,  are  exceedingly 
complex.  The  number  of  plant  forms  are  so  numerous  that 
the  discussion  already  presented  serves  as  little  more  than  an 
introduction.  In  the  second  place,  the  transformations  facili- 
tated by  soil  organisms  involve  all  of  the  normal  constituents 
of  the  soil,  both  organic  and  inorganic.  Moreover,  biological 
activities  determine  to  a  large  degree  the  efficacy  of  every 
addition,  natural  or  artificial,  made  to  the  land.  While  the 
cycles  generally  recognized  are  apparently  clear  cut,  the 
transformations  themselves  are  actually  involved  in  intrica- 
cies, which  man  will  probably  never  entirely  unravel. 

A  third  phase  of  outstanding  importance  is  the  relationship 
of  the  biological  activities  of  the  soil  to  the  nitrogen  prob- 
lem. Not  only  are  the  complex  nitrogenous  compounds  of  the 
soil  readily  made  available  to  higher  plants  by  soil  organisms, 
but  means  are  provided  whereby  considerable  nitrogen,  in- 
ert as  it  is,  may  be  wrested  from  the  atmosphere  and  forced 
into  activity  within  the  soil.  It  is  not  impossible  that  in  cer- 
tain favored  cases  150  pounds  of  nitrogen  to  the  acre  may 
be  yearly  added  to  the  soil  by  such  processes.  This  phase 
alone  is  worthy  of  the  most  careful  practical  study.  Obvi- 
ously no_system  of  soil  management  can  be  wholly  successful 
unless  full  advantage  is  taken  of  this  and  other  biological 
possibilities  of  the  land. 


CHAPTER  XXII 
COMMERCIAL  FERTILIZER  MATERIALS1 

While  the  use  of  animal  excrement  on  cultivated  soils  was 
practiced  as  far  back  as  systematic  agriculture  can  definitely 
be  traced,  the  earliest  record  of  the  use  of  mineral  salts  for  in- 
creasing the  yield  of  crops  was  published  in  1669  by  Sir 
Kenelm  Digby.2  He  says:  "By  the  help  of  plain  salt  petre, 
diluted  in  water,  and  mingled  with  some  other  fit  earthly 
substance,  that  may  familiarize  it  a  little  with  the  corn  into 
which  I  endeavored  to  introduce  it,  I  have  made  the  barrenest 
ground  far  outgo  the  richest  in  giving  a  prodigiously  plentiful 
harvest."  His  dissertation  does  not  however,  show  any  true 
conception  of  the  reason  for  the  increase  in  the  crop  through 
the  use  of  this  fertilizer.  In  fact,  the  lack  of  any  real  knowl- 
edge at  that  time  of  the  composition  of  the  plant  would  have 
made  this  impossible. 

In  1804,  de  Saussure,3  a  Frenchman,  called  attention,  for 
the  first  time  to  the  significance  of  the  ash  ingredients  of 
plants  not  only  showing  that  these  mineral  materials  were 

*The  following  general  references  may  prove  helpful: 

Hall,  A.  D.,  Fertilizers  and  Manure;  New  York,  1921. 

Halligan,  J.  E.,  Soil  Fertility  and  Fertilizers;  Easton,  Pa.,  1912. 

Van  Slyke,  L.  L.,  Fertilizers  and  Crops;  New  York,  1912. 

Fraps,    G.    S.,    Principles    of   Agricultural    Chemistry;    Easton,    Pa., 

Collins,  S.  H.,  Chemical  Fertilizers  and  Parasiticides;  New  York, 
1920. 

2  Digby,  Kenelm,  A  Discourse  Concerning  the  Vegetation  of  Plants; 
London,  1669. 

3  Saussure,  Theodore  de,  Becherches  Chimiques  sur  la  Vegetation; 
Paris,  1804. 

442 


COMMERCIAL  FERTILIZER  MATERIALS      443 


obtained  from  the  soil  but  pointing  out  that  they  were  ab- 
solutely essential  for  plant  growth.  Liebig,1  in  Germany,  at 
about  the  middle  of  the  nineteenth  century,  emphasized  still 
more  strongly  the  importance  of  minerals  to  plants,  refuting 
the  theory,  at  that  time  current,  that  plants  obtained  all  of 
their  carbon  from  the  soil  organic  matter.  While  he  showed 
the  importance  of  potash  and  phosphoric  acid  in  manures,  he 
failed  to  appreciate  the  value  of  nitrogenous  materials,  hold- 
ing that  the  soil  received  sufficient  ammonia  in  rain-water. 
The  true  conception  of  the  necessity  of  supplying  nitrogen 
in  some  form  was  definitely  established  in  an  experimental 
way  in  1857  by  Lawes,  Gilbert  and  Pugh  2  of  the  Rothamsted 
Experiment  Station,  England.  The  extreme  care  used  by 
these  investigators  caused  them  to  sterilize  the  soil  with  which 
they  were  working.  They  thus  failed  to  discover  the  utiliza- 
tion of  free  atmospheric  nitrogen  by  legumes.  This  phe- 
nomenon, so  important  in  practical  agriculture,  was  explained 
by  Hellriegel  and  Wilforth  in  1886. 

Between  1840  and  1850  Sir  John  Lawes  placed  the  manu- 
facture of  superphosphates  on  a  commercial  basis  by  treating 
bones  and  coprolites  with  sulfuric  acid.  At  about  this  time 
the  importation  into  Europe  of  Peruvian  guano  and  sodium 
nitrate  began.  The  commercial  fertilizers  industry,  which 
has  now  attained  such  importance  in  practical  agriculture, 
may  be  considered  as  dating  from  this  period. 

243.  Commercial  fertilizers. — Although  the  commercial 
fertilizer  industry  is  but  little  more  than  seventy  years  old, 
the  sale  of  fertilizers  in  this  country  at  the  present  time 
amounts  to  millions  of  dollars  annually.    Animal  refuse  and 

1  Liebig,  J.  Justus  von,  Principles  of  Agricultural  Chemistry  with 
Special  Beference  to  the  Late  Researches  Made  in  England;  London, 
1855.  Also,  Chemistry  in  Its  Applications  to  Agriculture  and  Physiology ; 
New  York,  1856. 

2  Lawes,  J.  B.,  Gilbert,  J.  H.,  and  Pugh,  E.,  On  the  Sources  of  the 
Nitrogen  of  Vegetation,  with  Special  Beference  to  the  Question  Whether 
Plants  Assimilate  Free  or  Uncombined  Nitrogen;  Rothamsted  Memoirs, 
Vol.  1,  No.  1,  1862. 


444        NATURE  AND  PROPERTIES  OF  SOILS 

phosphates  are  exported,  while  sodium  nitrate  and  potash 
salts  are  imported  in  large  amounts.  Fifty  per  cent.  o£  the 
fertilizers  sold  in  the  United  States  are  applied  in  the  south 
Atlantic  states  within  three  or  four  hundred  miles  of  the 
seaboard.  Nearly  one-half  of  the  remainder  is  purchased  by 
the  New  England  and  middle  Atlantic  states.  West  of  the 
Mississippi  River,  the  use  of  fertilizers,  especially  those  car- 
rying phosphoric  acid,  is  increasing  rapidly. 

The  primary  function  of  a  commercial  fertilizer  is  to  supply 
plant  nutrients  to  the  soil  in  such  a  form  that  the  plant  may 
be  directly  influenced  by  such  an  application.  The  secondary 
influences  of  fertilizers  may  be  beneficial  or  detrimental.  The 
exact  nature  of  the  secondary  influences  depends  on  the  par- 
ticular fertilizer  applied  and  especially  on  the  type  of  soil 
and  the  crop  management  in  vogue. 

Prepared  fertilizers,  as  found  on  the  market,  are  usually 
composed  of  a  number  of  ingredients.  Since  these  ingredi- 
ents are  the  carriers  of  the  nutrient  constituents,  and  since 
it  is  on  their  composition  and  solubility  that  the  value  of  a 
fertilizer  depends,  a  knowledge  of  the  properties  of  these 
materials  is  not  only  of  interest  to  every  one  who  uses  fer- 
tilizers but  is  also  a  valuable  aid  in  their  purchase. 

FERTILIZERS   USED   FOR   THEIR   NITROGEN 

Nitrogen  is  usually  the  most  expensive  constituent  of  ma- 
nure and  is  of  great  importance,  since  it  is  very  likely  to  be 
deficient  in  soils.  A  commercial  fertilizer  may  have  its  nitro- 
gen in  the  form  of  soluble  inorganic  salts  or  in  organic  com- 
bination. On  the  form  depends  to  a  certain  extent  the  agri- 
cultural value  of  the  nitrogen,  as  the  soluble  inorganic  salts 
are  very  readily  available  to  the  plant,  while  the  organic  forms 
must  pass  through  the  various  biological  processes  before 
the  plant  can  use  the  nitrogen  so  contained.  Only  the  best- 
known  fertilizer  carriers  need  receive  particular  attention 
here. 


COMMERCIAL  FERTILIZER  MATERIALS      445 

244.  Dried  blood  and  tankage.1 — Both  of  these  fertilizers 
are  packing-house  products.  The  former  is  obtained  by  dry- 
ing the  blood  from  the  slaughtering  pens.  It  comes  on  the 
market  as  a  homogeneous^  blackish  to  dark  greyish  material, 
often  slightly  moist  and  with  a  characteristic  odor.  Its  con- 
tent of  ammonia  (NH3)  ranges  from  10  to  16  per  cent.,  de- 
pending on  the  grade  of  the  fertilizer.  It  often  contains 
traces  of  phosphoric  acid  (P205).2 

Tankage  is  a  mixture  of  various  refuse  materials  from  the 
slaughter-houses,  such  as  blood,  hair,  scraps  of  meat,  and  hide 
and  bone.  It  is  generally  steam-cooked  and  part  of  the  gela- 
tin and  fat  removed.  It  is  variable  in  composition,  carrying 
from  5  to  10  per  cent,  of  NH3  and  from  3  to  8  per  cent,  of 
P205.  The  phosphoric  acid  is  contained  in  the  bone  and  is 
in  the  form  of  tricalcium  phosphate  [Ca3(P04)2].  Tankage 
is  easily  distinguished  from  blood  meal  by  its  heterogeneous 
character. 

When  added  to  a  soil,  both  blood  and  tankage  undergo  rapid 
decomposition,  ammonification,  and  finally  nitrification.  Such 
fertilizers  are,  therefore,  very  effective  in  the  late  spring  and 
summer.  For  early  application,  however,  a  material  such 
as  sodium  nitrate  is  much  better,  since  a  biological  transfor- 
mation is  unnecessary  in  order  that  it  may  be  immediately 
utilized  by  the  plants. 

245.  Other  organic  nitrogenous  fertilizers. — Below  will 
be  found  the  composition  of  a  number  of  other  organic  ma- 
terials that  have  been  or  are  still  used  as  fertilizers.  Only  two 
need  explanation.  Guano  consists  of  the  excrement  and  car- 
casses of  sea  fowls,  the  composition  depending  on  the  climate 
and  position  in  which  it  is  found.  Guano  from  an  arid  region 
contains  ammonia,  phosphoric  acid,  and  potash.  Under 
humid  conditions  only  the  phosphoric  acid  remains  in  any 

1Fry,  W.  Hv  Identification  of  Commercial  Fertilizer  Materials;  TJ.  S. 
Dept,  Agr.,  Bui.  97,  1914. 

2  The  composition  of  commercial  fertilizers  is  commonly  expressed  in 
terms  of  ammonia  (NH3),  phosphoric  acid  (P205),  and  potash  (K20). 


446        NATURE  AND  PROPERTIES  OP  SOILS 

amount.  Typical  guano  carries  uric  acid,  urates,  and  am- 
monium salts.  The  phosphorus  occurs  as  calcium,  potas- 
sium, and  ammonium  phosphates.  The  potash  is  found  in 
the  chloride,  sulfate  and  phosphate  forms.  While  guano 
was  once  a  very  important  fertilizer,  the  deposits  are  very 
nearly  exhausted  and  but  little  now  appears  on  the  market. 
Process  fertilizers  are  obtained  by  treating  organic  trade 
wastes  and  refuse  with  acid  or  with  steam  under  pressure 
Hydrolysis  of  the  proteins  occurs  with  the  formation  of  pro- 
teoses, peptones,  and  simple  amino  acids.  The  water  soluble 
nitrogen  of  such  materials  has  been  shown  by  Lathrop  of  the 
United  States  Bureau  of  Soils  to  be  as  readily  available  as 
that  of  dried  blood  or  tankage. 


Table  XCV 

Fertilizer 

NH, 

P,0, 

K,0 

Guano 

10-14 
1-  3 

10-13 
8-11 
.8^12 

10-16 
8k10 

4r-   6 

5-  7 

6-  7 

10-12 

6-  7 

1-  2 
Ir-  2 
1-  1% 

2-5 

Process  goods 

Hoof  meal 

Fish  scrap 

Leather  meal 

Wool  and  hair  waste 

Cottonseed   meal 

Linseed  meal 

2-3 
1-2 

Castor  pomace 

3-11/2 

These  compounds  vary  greatly  in  their  values  as  fertilizers. 
Guano,  process  goods,  and  fish  scrap  when  in  the  soil  decom- 
pose rapidly  and  are  as  effective  ordinarily  as  blood  or  tank- 
age. Untreated  leather  meal  and  wool  and  hair  waste  decay 
very  slowly  and  are  of  little  value  as  fertilizing  materials. 

246.  Utilization  of  nitrogenous  organic  compounds  by 
plants. — One  of  the  early  beliefs  in  regard  to  plant  nutrition 
was  that  organic  matter  as  such  is  directly  absorbed  by  higher 
plants.    This  opinion  was  afterwards  entirely  replaced  by  the 


COMMERCIAL  FERTILIZER  MATERIALS       447 

mineral  theory  propounded  by  Liebig  j  and  still  later  the  dis- 
covery of  the  nitrifying  process  almost  disposed  completely  of 
the  belief  that  organic  matter  is  used  directly  by  higher 
plants.  It  is  quite  certain,  however,  that  some  organic  nitrog- 
enous compounds  furnished  suitable  material  for  some  higher 
plants  without  undergoing  bacterial  change  and  producing 
a  nitrate  form  of  nitrogen. 

The  following  compounds  have  been  shown  by  Hutchinson 
and  Miller1  to  be  readily  assimilated  by  peas:  acetamide, 
urea,  barbituric  acid,  and  alloxan.  Formamide,  glycerine, 
cyanuric  acid,  oxamine,  peptone,  and  sodium  aspartate  were 
assimilated  but  less  easily.  Creatinine  has  been  shown  by 
Skinner  2  to  be  used  directly  by  plants  as  a  source  of  nitro- 
gen. Histidine,  arginine,  and  creatine  have  also  been  found 
in  soils  and  it  has  been  demonstrated  that  they  have  a  direct 
influence  on  wheat  seedlings. 

These  and  numerous  other  investigations  of  this  subject 
show  that  amine  as  well  as  amide  nitrogen  is  assimilated  by 
at  least  some  agricultural  plants,  but  to  what  extent  most 
of  these  compounds  may  successfully  replace  the  inorganic 
forms  of  nitrogen,  such  as  the  nitrates,  has  not  been  definitely 
established.  Certain  organic  nitrogenous  fertilizers — as,  for 
example,  dried  blood — have  a  high  commercial  value,  the 
nitrogen  in  this  form  selling  for  more  a  pound  than  the  nitro- 
gen in  any  of  the  inorganic  salts.  Many  crops,  especially  cer- 
tain vegetables,  are  most  successfully  grown  only  when 
supplied  with  organic  nitrogenous  material.  Some  ni- 
trate nitrogen  is  always  present  under  natural  soil  condi- 
tions, so  that  crops  are  never  limited  to  organic  nitrogen 
alone ;  and  it  may  be  that  the  latter  form  of  nitrogen  is  most 
useful  when   it  supplements  the  nitrate  form. 

1  Hutchinson,  H.  B.,  and  Miller,  N.  H.  J.,  The  Direct  Assimilation 
of  Inorganic  and  Organic  Forms  of  Nitrogen  by  Higher  Plants;  Centrlb. 
f.  Bakt.,  II,  Band  30,  Seite  513-547,  1911. 

2  Skinner,  J.  J.,  III.  Effects  of  Creatinine  on  Plant  Growth;  U.  S. 
Dept.  Agr.,  Bur.  Soils,  Bui.  83,  pp.  33-44,  1911. 


448        NATURE  AND  PROPERTIES  OF  SOILS 

247.  Sodium  nitrate  (NaN03  +).1— Sodium  nitrate  is 
mined  in  Chile,  occurring  as  a  crude  salt  (caliche)  in  the 
semiarid  regions  along  the  coast.  It  is  found  near  the  sur- 
face under  an  over  burden  of  varying  thickness.  The  cal- 
iche contains,  besides  sodium  nitrate,  such  salts  as  NaCl, 
K2S04,  Na2S04,  and  MgS04  besides  traces  of  Na2COs,  K2C08, 
and  boron.  The  refined  salt,  which  is  shipped  to  this  country, 
carries  from  2  to  3  per  cent,  of  NaCl  and  KN03.  Its  am- 
monium content  is  generally  rated  at  about  18  per  cent. 

The  fertilizer  appears  on  the  market  in  clouded  crystals  of 
a  yellowish  cast,  extremely  soluble  in  water  and  quite  de- 
liquescent. The  fertilizer  is  generally  alkaline  to  litmus. 
In  the  soil  it  diffuses  rapidly  and  is  immediately  avail- 
able to  plants.  For  this  reason  it  is  extremely  valuable  early 
in  the  spring  before  nitrification  is  active. 

The  long-continued  use  of  sodium  nitrate  will  tend  to  pro- 
duce an  alkaline  residue  of  sodium  carbonate  in  the  soil.2 
This  is  due  to  the  absorptive  power  of  the  soil  for  sodium  and 
the  ease  with  which  the  nitrate  ions  are  lost  in  drainage.  The 
plant,  by  using  large  amounts  of  nitrates,  intensifies  this  se- 
lective absorption. 

The  origin  of  the  caliche  deposits  is  problematical.  The 
theory  has  been  advanced  that  the  origin  is  due  to  the  de- 
composition of  great  deposits  of  seaweed  on  an  uplifted  con- 
tinental shelf.  Another  hypothesis  would  have  the  deposits 
originate  from  wind-carried  guano  dust.  As  rational  a  the- 
ory as  any  is  proposed  by  Singewald  and  Miller,3  who  believe 
the  nitrates  were  leached  from  the  Andes  Mountains  and 

1  Fertilizer  materials  are  never  pure  salts.  The  plus  after  the  formula 
indicates  the  presence  of  impurities. 

2  Hall,  A.  D.,  The  Effect  of  the  Long  Continued  Use  of  Sodium  Nitrate 
on  the  Constitution  of  the  Soil;  Trans.  Chem.  Soc.  (London),  Vol.  85, 
pp.  950-971,  1904.  Also,  Brown,  B.  E.,  Concerning  Some  Effects  of  Long- 
Continued  Use  of  Sodium  Nitrate  and  Ammonium  Sulfate  on  the  Soil; 
Ann.  Eep.  Pa.  State  Coll.,  1908-1909,  pp.  85-104. 

3  Singewald,  J.  1ST.,  and  Miller,  B.  L.,  Genesis  of  the  Chilean  Nitrate 
Deposits;  Econ.  Geol.,  Vol.  II,  pp.  103-113;  1916. 


COMMERCIAL  FERTILIZER  MATERIALS       449 

carried  by  ground  water  to  their  present  location.  The  con- 
centration of  the  salts  is  considered  by  these  authors  as  due 
to  surface  evaporation  and  consequent  upward  capillary 
movement  of  the  highly  charged  ground  water. 

248.  Ammonium  sulfate  ((NH4)2S04  +  ).— This  fertil- 
izer is  a  by-product  from  coke  ovens  and  from  the  distilla- 
tion of  coal  in  gas  manufacture.1  About  one-fifth  of  the 
nitrogen  of  the  coal  is  thus  driven  off  as  ammonia,  which  is 
caught  in  special  washing  devices.  The  mother  liquid  is  then 
distilled,  the  NH3  being  driven  into  sulfuric  acid.  The  prod- 
uct is  later  concentrated  and  the  salt  crystallized  out.  An- 
other and  simpler  process  provides  for  a  direct  union  of  the 
gas  and  the  acid,  thus  eliminating  the  washers. 

This  fertilizer  usually  carries  about  25  per  cent,  of  am- 
monia. It  usually  has  a  greyish  or  greenish  color  due  to 
coal-tar  products.  This  commercial  ammonium  sulfate  is 
very  soluble  in  water  and  has  a  characteristic  taste.  When 
heated,  it  readily  breaks  up,  giving  off  ammonia  gas.  It 
is  very  acid  to  litmus  paper,  due  to  the  union  of  a  weak 
base  with  a  strong  acid  radical.  The  ammonia  is  very  strongly 
absorbed  by  the  soil  and  also  is  used  to  a  greater  extent  by 
the  plant  than  are  the  sulfate  ions.  It  thus  leaves  in  the 
soil  an  acid  residue2  which  should  be  alleviated  by  lime  if 
the  soil  is  not  already  supplied  with  plenty  of  active  calcium 
and  magnesium.  In  a  warm  soil  the  ammonia  is  quickly 
nitrified  to  the  nitrate  form.    This  transformation  is  general- 

1  By-Product  CoTce  and  Gas  Plants;  The  Koppers  Company,  Pitts- 
burgh. 

Sulfate  of  Ammonia.  Its  Source,  Production  and  Use;  The  Barrett 
Company,  New  York. 

2  Hall,  A.  D.,  and  Gimingham,  C.  T.,  The  Interaction  of  Ammonium 
Salts  and  the  Constitution  of  the  Soil;  Jour.  Chem.  Soc.  (London), 
Vol.  91,  pt.  1,  p.  677,  1907. 

White,  J.  W.,  The  Besults  of  Long  Continued  Use  of  Ammonium 
Sulfate  Upon  a  Besidual  Limestone  Soil  of  the  Hagerstown  Series;  Ann. 
Kep.  Pa.  State  Coll.,  1912-1913,  pp.  55-104. 

Ruprecht,  R.  W.r  and  Morse,  F.  W.,  The  Effect  of  Sulfate  of  Ammonia 
on  Soil;  Mass.  Agr.  Exp.  Sta.,  Bui.  165,  1915. 


450        NATURE  AND  PROPERTIES  OF  SOILS 

ly  so  rapid  as  to  make  this  fertilizer  almost  as  quickly  effec- 
tive as  sodium  nitrate. 

While  the  nitrogen  of  ammonium  salts  is  quickly  changed 
to  the  nitrate  combination  in  a  well-drained  soil,  some  plants 
seem  to  prefer  ammoniacal  nitrogen  to  the  nitrate  form.  Kell- 
ner 1  in  1884  and  later  Kelley 2  demonstrated  that  rice  plants 
growing  on  lowland  soils  use  ammoniacal  nitrogen  rather 
than  other  forms.  On  upland  soils,  however,  it  is  presumable 
that  rice  plants  utilize  nitrate  nitrogen,  which  would  indi- 
cate that  some  plants,  at  least,  may  adapt  themselves  to  the 
use  of  a  more  abundant  form  of  nitrogen. 

Hutchinson  and  Miller 3  found  that  peas  obtained  nitrogen 
from  ammonium  salts  as  readily  as  from  sodium  nitrate,  but 
that  wheat  plants,  although  able  to  obtain  nitrogen  directly 
from  ammonium  salts,  grew  much  better  in  a  solution  con- 
taining nitrates.  One  feature  brought  out  by  the  numerous 
experiments  with  ammonium  salts  is  the  difference  between 
plants  of  various  kinds  in  respect  to  their  ability  to  absorb 
nitrogen  in  this  form. 

249.  The  artificial  fixation  of  nitrogen.4 — The  vast  store 
of  atmospheric  nitrogen,  chemically  uncombined  and  very 
inert,  will  furnish  an  inexhaustible  supply  for  plants  when  it 
can  with  reasonable  economy  be  combined  in  some  manner  to 
give  a  product  that  can  be  commercially  transported  and 
that  will,  when  placed  in  the  soil,  become  available  without 
liberating  substances  toxic  to  plants.     The  importance  of  the 

1Kellner,  O.,  Agrikulturchemische  Stvdien  uber  die  Beislcultur ; 
Landw.  yers.  Stat.,  Band  30,  Seite  18-41,  1884. 

2  Kelley,  W.  P.,  The  Assimilation  of  Nitrogen  by  Bice;  Haw.  Agr.  Exp. 
Sta.,  Bui.  24,  pp.  5-20,  1911. 

3  Hutchinson,  H.  B.,  and  Miller,  N.  H.  J.,  The  Direct  Assimilation  of 
Inorganic  and  Organic  Forms  of  Nitrogen  by  Higher  Plants;  Centrlb. 
f.  Bakt.,  II,  Band  30,  Seite  513-547,  1911. 

4  Norton,  T.  H.,  Utilization  of  Atmospheric  Nitrogen;  U.  S.  Dept.  of 
Comm.  and  Labor,  Special  Agents  Ser.,  No.  52,  1912. 

Knox,  J.,  Fixation  of  Atmospheric  Nitrogen;  New  York. 
Slosson,  E.  E.,  Creative  Chemistry,  Chaps.  II  and  III:    New  York, 
1920. 


COMMERCIAL  FERTILIZER  MATERIALS       451 

nitrogen  supply  for  agriculture  may  be  appreciated  when  it 
is  considered  that  nitrates  are  being  carried  off  in  the  drain- 
age water  of  all  cultivated  lands  at  a  surprisingly  rapid  rate. 
A  Dunkirk  silty  clay  loam  at  Cornell  University,1  carrying 
a  rotation  of  maize,  oats,  wheat,  and  hay,  lost  in  crop  and 
drainage  water  in  a  period  of  ten  years  over  77  pounds  to  the 
acre  of  nitrogen  annually.  This  is  equivalent  to  520  pounds 
of  commercial  sodium  nitrate  or  to  about  380  pounds  of  com- 
mercial ammonium  sulfate. 

The  exhaustion  of  the  supply  of  nitrogen  in  most  soils  may 
be  accomplished  within  one  or  two  generations,  unless  a  re- 
newal of  the  supply  is  brought  about  in  some  way.  Natural 
processes  provide  for  an  annual  accretion  through  the  wash- 
ing-down of  ammonia  and  nitrates  by  rain-water  from  the 
atmosphere,  and  through  the  fixation  of  free  atmospheric 
nitrogen  by  bacteria.  Farm  practice  of  the  present  day  re- 
quires the  application  of  nitrogen  in  some  form  of  manure, 
and,  as  the  end  of  the  commercial  supply  of  combined  nitro- 
gen is  easily  in  sight,  there  is  urgent  need  of  discovering  a 
new  source.  The  world  war  has  given  great  impetus  to  the 
study  of  the  artificial  fixation  of  nitrogen  and  a  number  of 
compounds  thus  produced  are  on  the  market  or  will  appear 
shortly. 

250.  Calcium  cyanimid  (CaCN2+).2 — The  manufacture 
of  this  fertilizer  begins  with  calcium  carbide  (CaC2)  which 
is  produced  by  heating  lime  and  coke  together. 

CaO  +  3C  =  CaC2  +  CO 

This  impure  carbide  is  then  powdered  and  heated  elec- 
trically in  special  ovens.  At  the  proper  temperature  nitro- 
gen gas  is  passed  through  the  carbide  with  the  following  re- 
sult: 

CaC2  +  N2  =  CaCN2  +  C 

*For  complete  data,  see  par.  163,  this  text. 
aPranke,  E.  J.,  Cyanamid;  Easton,  Pa.,  1913. 


452        NATURE  AND  PROPERTIES  OF  SOILS 

The  product  is  a  black  dry  crystalline  powder  of  rather 
light  weight,  containing  about  20  per  cent,  of  NH3.  It  is  very 
impure  as  shown  by  the  following  analysis: 

CaCN2    45.9  C     13.1 

CaC03    4.0  Fe203  and  A1.,03 1.9 

CaS     1.7  Si02    1.6 

CaaP2     1  MgO   1 

Ca(0H)2   26.6  H20    3 

Its  odor  and  the  presence  of  carbon  are  characteristic.  It 
is  intensively  alkaline  to  litmus.  In  the  soil  it  undergoes  a 
number  of  very  complex  changes,  urea  ultimately  being  pro- 
duced. Toxic  compounds  are  present  as  the  reactions  pro- 
ceed. It  should,  therefore,  be  placed  in  the  soil  some  time  be- 
fore the  crop  is  seeded.  The  carbon  seems  to  aid  in  the  trans- 
formation as  a  catalytic  agent.  The  urea  quickly  breaks 
down  biologically  to  ammonia : 

CON2H4  +  2H20  =  (NH4)2  C03 

This  ammonia  is  then  oxidized  to  the  nitrate  form. 

251.  Basic  calcium  nitrate  (Ca(N03)2+).— This  fertil- 
izer, like  calcium  cyanimid,  is  produced  by  the  artificial  fixa- 
tion of  nitrogen.  Air  is  passed  through  an  electric  arc  of  high 
temperature.  Under  such  conditions  a  part  of  the  oxygen  and 
the  nitrogen  are  forced  together  forming  nitric  oxide.  This 
gas  is  then  oxidized  in  suitable  chambers  to  the  peroxide, 
which  is  passed  into  water,  producing  nitric  acid.  The  nitric 
oxide  which  also  results  is  led  back  to  the  oxidizing  chambers. 

The  reactions  are  as  follows : 

N2  +  02  =  2NO 

2NO  +  02  =  2N02 

3N02  +  H20  =  2HN03  +  NO 

The  nitric  acid  is  passed  into  lime-water,  giving  calcium 
nitrate.  This  fertilizer  contains  from  13  to  16  per  cent,  of 
ammonia  and  is  intensely  alkaline  to  litmus.    Due  to  its  high 


COMMERCIAL  FERTILIZER  MATERIALS      453 

deliquescence,  it  must  either  be  treated  in  some  way,  which 
raises  the  cost  of  manufacture,  or  must  be  shipped  in  sealed 
casks.  It  is  very  soluble  in  water  and  is  immediately  available 
to  plants.    It  leaves  no  harmful  residue  in  the  soil. 

252.  Other  methods  of  nitrogen  fixation. — Calcium  ni- 
trate, because  of  its  cost,  cannot  compete  either  with  sodium 
nitrate  or  ammonium  sulfate  and  is  not  manufactured  in  this 
country.  Calcium  cyanamid  is  produced  only  in  amounts 
sufficient  to  satisfy  the  demands  of  mixed  fertilizer  manu- 
facture. Its  dry  character  makes  it  valuable  in  such  com- 
pounding. 

At  the  present  time  a  number  of  more  efficient  methods  of 
artificially  fixing  nitrogen  are  known.  The  Haber  process 
proved  extremely  successful  in  Germany,  especially  when 
supplemented  by  the  Oswald  method  of  converting  ammonia 
into  nitric  acid.  In  the  Haber  method  a  mixture  of  nitrogen 
and  hydrogen  are  placed  under  pressure  and  moderately 
heated  in  the  presence  of  a  catalyst.  A  good  yield  of  ammonia 
results. 

N2  +  3H2  =  2NH3 

In  the  Oswald  method  this  ammonia  is  passed  over  a  cata- 
lytic agent  in  the  presence  of  oxygen. 

NH3  +  202  =  HN03  +  H20 

The  advantage  of  producing  both  ammonia  and  nitric  acid 
is  obvious,  as  ammoniun  nitrate  (NH4N03),  ammonium  phos- 
phate ((NH4)3P04),  and  potassium  nitrate  (KN03)  may  be 
produced  at  one  plant. 

During  the  war  Professor  Bucher  of  Brown  University  per- 
fected a  simple  and  inexpensive  method  of  producing  sodium 
cyanide  synthetically.  Producers  gas,  formed  by  passing  air 
over  hot  coal,  is  forced  through  a  heated  revolving  drum  con- 
taining soda  ash,  iron,  and  coke.    The  reaction  is  as  follows: 

Na2C03  +  4C  +  N2  =  2NaCN  +  3CO 


454         NATURE  AND  PROPERTIES  OF  SOILS 

Ammonia  may  be  produced  very  easily  from  the  sodium 
cyanide  and  used  as  such  or  changed  to  nitric  acid  by  the 
Oswald  method. 

253.  Relative  availability  of  nitrogen  fertilizers.1— It  is 
very  difficult  to  rank  nitrogenous  fertilizers  on  the  basis  of 
their  rate  of  availability,  since  the  conditions  within  the  soil 
so  markedly  influence  the  transformations,  especially  those 
of  a  biological  nature.  Dried  blood  and  ammonium  sulfate, 
for  example,  will  give  almost  as  quick  results  in  a  warm,  well 
aerated  soil,  as  far  as  higher  plants  are  concerned,  as  sodium 
nitrate.  In  general,  however,  the  nitrate  fertilizers  should  be 
rated  as  most  readily  available,  followed  in  order  by  ammo- 
nium salts,  dried  blood,  tankage,  and  similar  materials.  Such 
substances  as  wool,  hair,  and  untreated  leather  waste  should 
rank  last. 

FERTILIZERS   USED  FOR   THEIR   PHOSPHORUS 

Phosphorus  is  generally  present  in  nature  in  combination 
with  calcium,  iron,  or  aluminum.  Some  phosphates  carry  or- 
ganic matter  and  when  thus  associated  are  generally  consid- 
ered to  decompose  more  readily  when  added  to  the  soil. 

254.  Bone  phosphate  (Ca3(P04)2+).— Bones  were  for- 
merly applied  to  the  soil  in  the  raw  condition,  either  ground 
or  unground.  Most  bone  as  now  sold  is  merely  steamed  or 
boiled  to  remove  the  fat  and  nitrogenous  matter,  which  is 
used  in  other  ways.  Bone-meal  comes  on  the  market  as  a  dusty 
powder  of  characteristic  odor.  It  contains  about  27  per  cent, 
of  phosphoric  acid  as  tricalcium  phosphate.  Tankage,  which 
has  already  been  spoken  of  as  a  nitrogenous  fertilizer,  con- 
tains from  3  to  8  per  cent,  of  phosphoric  acid,  largely  in  the 
form  of  tricalcium  phosphate.  All  bone  phosphates  are  slow- 
acting  manures,  and  should  be  used  in  a  finely  ground  form 
and  for  the  permanent  benefit  of  the  soil  rather  than  as  an 

J  Thome,  C.  E.,  Carriers  of  Nitrogen  in  Fertilizers;  Soil  Sci.,  Vol. 
IX,  No.  6,  pp.  487-494,  1920. 


COMMERCIAL  FERTILIZER  MATERIALS       455 

immediate  source  of  phosphorus.    In  the  soil,  water  charged 
with  carbon  dioxide  slowly  converts  the  insoluble  tricalcium 
phosphate  into  the  soluble  mono-calcium  form: 
Ca3(POJ2  +  4C02  +  4H20  =  CaH4(P0J2  +  2CaH2(C03)2 

255.  Rock  phosphate1  (Ca3(Po4)2-}-). — There  are  many 
natural  deposits  of  mineral  phosphates  in  different  parts  of 
the  world,  some  of  the  most  important  of  which  are  in  North 
America.  The  phosphorus  in  all  of  these  is  in  the  form  of 
tricalcium  phosphate,  but  the  materials  associated  with  it 
vary  greatly.  Rock  phosphate  may  occur  in  nature  as  soft 
phosphate,  pebble  phosphate,  boulder  phosphate,  and  hard 
rock  phosphate. 

South  Carolina  phosphate  contains  from  26  to  28  per  cent, 
of  phosphoric  acid  and  a  very  small  amount  of  iron  and 
aluminum.  As  these  latter  substances  interfere  with  the  man- 
ufacture of  acid  phosphate  from  rock,  their  presence  is  very 
undesirable,  rock  containing  more  than  from  3  to  6  per  cent, 
being  unsuitable  for  that  purpose. 

Florida  phosphates  exist  in  the  form  of  soft  phosphate, 
pebble  phosphate,  and  boulder  phosphate.  Such  phosphate 
contains  from  18  to  30  per  cent,  of  phosphoric  acid,  and  be- 
cause of  its  being  softer  than  most  of  these  rocks  it  is  often 
applied  to  the  land  without  being  first  converted  into  a  soluble 
form.  The  other  two  forms,  pebble  phosphate  and  boulder 
phosphate,  are  highly  variable  in  composition,  ranging  from 
20  to  40  per  cent,  in  phosphoric  acid  content.  Tennessee 
phosphate,  which  is  now  very  important,  contains  from  25  to 
35  per  cent,  of  phosphoric  acid. 

Rock  phosphate,  or  floats  as  it  is  often  called,  appears  on 

the  market  as  a  heavy  finely  ground  powder  of  light  gray 

color.    It  generally  carried  about  27  per  cent,  of  phosphoric 

acid  as  Ca3(P04)2.    A  typical  analysis  is  as  follows: 

1Waggaman,  W.  Hv  and  Fry,  W.  H.,  Phosphate  Rock  and  Methods 
Proposed  for  Its  Utilisation  as  a  Fertiliser;  U.  S.  Dept.  Agr.,  Bui.  312, 
1915. 


456        NATURE  AND  PROPERTIES  OF  SOILS 

Moisture,  organic  matter,  etc 5.06 

Ca3(P04)2     77.76 

FeP04  and  A1P04 1.50 

CaC03    4.43 

MgC03  50 

CaF2  +  CaCl,     6.11 

FeS 77 

Fe203  and  A1203   3.87 

Rock  phosphate  undergoes  the  same  change  in  the  soil  as 
bone-meal  but  generally  much  more  slowly,  unless  the  soil  is 
very  high  in  organic  matter.  Mixing  the  rock  with  manure 
seems  to  hasten  its  availability  to  plants. 

256.  Acid  phospthate x  (CaH4(P04)2+).— Acid  phosphate 
is  a  dry  material  of  a  browning  gray  color,  partially  soluble  in 
water,  and  has  a  characteristic  acrid  odor.  It  is  intensely 
acid  to  litmus,  as  it  contains  certain  acid  salts.  It  carries 
from  14  to  16  per  cent,  of  available  P205  and  small  amounts 
of  insoluble  P205.  It  is  made  by  treating  raw  rock  with  sul- 
furic acid  under  the  proper  conditions.2 

Ca3(P04)2  +  2H2S04  =  CaH4(POJ2  +  2Ca  S04 
( insoluble )  ( water  soluble ) 

The  acid  is  never  added  in  amounts  capable  of  quite  com- 
pleting this  reaction.  Some  di-calcium  phosphate  [Ca2H2 
(P04)2],  spoken  of  as  citrate  soluble  or  reverted  phosphoric 
acid,  is  thus  produced. 

Ca3(P04)2  +  H2S04  =  Ca2H2(P04)2  +  CaS04 
(insoluble)  (reverted) 

Acid  phosphate  consists  mostly  of  gypsum  and  mono-cal- 
cium phosphate  with  some  di-calcium  phosphate  and  impuri- 

1  Chemically,  three  forms  of  phosphoric  acid  are  recognized  by  the 
fertilizer  industry:  (1)  insoluble  (CaB(P04)2),  (2)  reverted  or  citrate 
soluble  (CaaHa(P04)a),  and  (3)  water  soluble  (CaH4(P04)2).  The 
water  soluble  and  citrate  soluble  phosphates  are  rated  as  available  to 
plants.    The  insoluble  form  is  considered  as  unavailable. 

2  Waggaman,  W.  H.,  The  Manufacture  of  Acid  Phosphate;  U.  S.  Dept. 
Agr.,  Bui.  144,  1914. 


COMMERCIAL  FERTILIZER  MATERIALS      457 

ties.    The  water  soluble  and  reverted  phosphoric  acid  are  both 
rated  as  available. 

The  phosphates  of  acid  phosphate  when  added  to  the  soil 
quickly  revert  to  an  insoluble  form: 

CaH4(P04)2  +  2CaH2(C03)2  =  Ca3(P04)2+4C02  +  4H20 
Ca2H2(P04)2  +  CaH2(CO,)2  =  Ca8(POJ2  +  2C02  +  2H20 

Plenty  of  active  calcium  should  be  present  when  acid  phos- 
phate is  used  to  insure  this  reaction  instead  of  the  formation 
of  the  very  insoluble  ferric  phosphate  (FeP04)  and  aluminum 
phosphate  (A1P04).  Acid  phosphate  does  not  seem  to  make 
the  soil  acid.1  In  fact,  it  is  considered  by  some  investigators 
to  decrease  the  acidity  by  rendering  aluminum  and  iron  in- 
soluble. 

257.  Basic  slag  ( (CaO)5.P205.Si02+).— Iron  or  steel  con- 
taining over  2  per  cent,  of  phosphorus  is  too  brittle  to  be 
useful  and,  as  a  consequence,  ores  of  this  character  were  little 
used  until  methods  of  removing  this  phosphoric  acid  were 
discovered.  The  use  of  wood  in  smelting  provided  a  basic  ash, 
thus  removing  phosphorus  from  the  pig  iron.  With  coal,  how- 
ever, the  slag  is  acid  and  the  phosphorus  remains  with  the 
ore.  In  the  open-hearth  method  of  smelting  the  furnaces  are 
lined  with  a  specially  prepared  dolomitic  limestone.  Lime  is 
later  added  as  the  smelting  proceeds.  The  calcium  of  the 
slag  unites  with  the  phosphorus  of  the  iron,  thus  reducing 
the  percentage  of  that  element  in  the  steel.  The  most  prob- 
able formula  for  the  phosphorus  compound  in  basic  slag  is 
(€aO)5.P205.Si02.  Basic  slag  contains  a  large  amount  of 
iron  and  calcium  hydroxide.    Below  is  a  typical  analysis : 

CaO    45.0  A1203    1.7 

MgO    6.2  Si02 6.9 

FeO  +  Fe203    17.6  P205    18.1 

MnO   3.5  Other    constituents. .  .   1.0 

1  Conner,  S.  D.,  Acid  Soils  and  the  Effect  of  Acid  Phosphate  and 
Other  Fertilizers  Upon  Them;  Jour.  Ind.  and  Eng.  Chem.,  Vol.  8,  No. 
1,  pp.  35-40,  1916. 


458         NATURE  AND  PROPERTIES  OF  SOILS 

Basic  slag  comes  on  the  market  as  a  heavy  dark  gray  pow- 
der, extremely  alkaline  to  litmus,  and  contains  from  14  to 
20  per  cent,  of  P205.  The  phosphorus  of  basic  slag  is  almost 
all  soluble  in  citric  acid  and,  therefore,  is  rated  as  available 
phosphoric  acid.  It  does  not  revert  in  the  soil  as  does  acid 
phosphate,  but  is  immediately  attacked  by  carbon  dioxide  and 
rendered  rather  quickly  available.  A  possible  reaction  is  as 
below : 

(CaO)6.P206.Si02  +  8C02  +  6H20  =  CaH<(P04)2  + 
4CaH2(C08)2  +  Si02 

258.  Relative  availability  of  phosphate  fertilizers. — 
Acid  phosphate  carries  most  of  its  phosphoric  acid  in  a  water- 
soluble  form  and  although  the  phosphates  revert  to  the  tri- 
calcium  form  immediately  when  added  to  the  soil,  they  are 
rather  readily  available  to  plants.  This  is  due  to  the  charac- 
ter of  the  freshly  precipitated  salt  and  the  surface  exposed 
for  solution  activities.  To  insure  a  good  distribution  in  the 
soil  of  the  phosphoric  acid  and  a  rapid  influence  on  crops,  acid 
phosphate  should  be  well  mixed  with  the  soil. 

Basic  slag,  since  its  phosphoric  acid  is  largely  citrate  sol- 
uble, is  generally  considered  as  next  to  acid  phosphate  in 
availability.  Steamed  bone-meal  usually  gives  better  results 
than  raw  rock  phosphate  and  rates  third,  with  rock  phos- 
phate fourth  in  availability.  The  degree  of  fineness  makes  a 
great  difference  in  the  availability  of  the  less  soluble  phos- 
phate fertilizers,  especially  the  ground  bone  and  raw  rock 
phosphate.  The  latter  material  should  be  ground  fine  enough 
to  pass  through  a  sieve  having  at  least  one  hundred  meshes  to 
the  inch. 

259.  Raw  rock  phosphate  versus  acid  phosphate. — Con- 
siderable discussion  as  well  as  controversy  has  of  late  arisen 
regarding  the  relative  merits  of  acid  phosphate  and  raw  rock 
phosphate  not  only  when  applied  on  the  basis  of  equal  amounts 
of  phosphoric  acid  but  also  when  compared  on  the  basis  of 


COMMERCIAL  FERTILIZER  MATERIALS       459 

equal  money  values.  If  rock  phosphate  could  be  made  to 
equal  or  nearly  equal  the  availability  of  acid  phosphate,  ob- 
vious advantages  would  accrue,  since  raw  rock  costs  much  less 
than  acid  phosphate  and  carries  about  twice  as  much  phos- 
phoric acid. 

The  availability  of  the  phosphorus  of  raw  rock  phosphate 
varies  considerably  with  conditions.  At  least  four  major  in- 
fluences have  been  recognized:  (1)  the  character  of  the  crop 
grown,  (2)  reaction  of  the  soil,  (3)  the  character  of  accom- 
panying salts,  and  (4)  the  decomposition  of  organic  matter. 
It  is  to  be  expected  that  the  various  kinds  of  plants  should 
not  be  equally  influenced  by  the  phosphorus  of  tri-calcium 
phosphate.  Prianischnikov *  found  that  lupines,  mustard, 
peas,  buckwheat,  and  vetch  responded  to  fertilization  with 
raw  rock  phosphate  in  the  order  named,  while  the  cereals 
did  not  respond  at  all.  He  did  not  include  maize  in  his  ex- 
periments, but  that  crop  is  said  to  respond  well  to  difficultly 
soluble  phosphates.  It  is  generally  considered  that  those 
plants  which  have  a  long  growing  season  are  better  able  to 
utilize  tri-calcium  phosphate  than  are  more  rapidly  growing 
plants. 

A  number  of  investigators  have  stated,  as  a  result  of  their 
experimentation,  that  the  availability  of  raw  rock  phosphate 
is  greater  in  acid  soils  than  in  those  strongly  basic.  If  acidity 
of  the  soil  is  due  to  the  presence  of  an  actual  acid,  it  is  con- 
ceivable that  the  availability  may  be  due  to  the  solvent  action 
of  the  soil  acid  on  the  calcium  of  the  tri-calcium  phosphate, 
producing  the  di-calcium  salt  which  appears  to  be  fairly  read- 
ily available  to  plants.  When,  however,  soil  acidity  is  due 
to  a  lack  of  certain  active  bases,  the  case  is  different.    Gedroiz 2 

1  Prianisehnikov,  D.,  Bericht  uber  Verschiedene  Versuehe  mit  Bohphos- 
phaten  unter  Beduction ;  Moscow,  1910. 

"Gedroiz,  K.  K.,  Soils  to  which  BocJc  Phosphates  May  Be  Applied 
with  Advantage;  Jour.  Exp.  Agron.  (Russian),  Vol.  12,  pp.  529-539, 
811-816,  1911.  The  authors  are  indebted  to  Dr.  J.  Davidson  for  the 
translation. 


460        NATURE  AND  PROPERTIES  OF  SOILS 

explains  this  on  the  basis  of  the  absorptive  properties  of  the 
so-called  acid  soil.  He  regards  rock  phosphate,  not  as  a  chemi- 
cal compound,  but  as  a  solid  solution  of  di-calcium  phosphate 
with  lime.  According  to  Gedroiz  it  is  this  excessive  basicity 
of  the  phosphate  which  is  responsible  for  its  unavailability. 
Absorption  of  the  excess  calcium  would  leave  the  phosphate 
in  a  more  readily  available  condition  by  forming  the  di- 
calcium  salt. 

The  presence  of  certain  salts  has  been  found  to  influence  the 
availability  of  difficultly  soluble  phosphates.  The  subject  has 
been  investigated  by  a  large  number  of  experimenters,  and  it 
will  be  possible  to  summarize  their  results  only  in  part  and 
very  briefly.  It  has  been  found,  for  example,  that  calcium 
carbonate  decreases  the  availability  of  raw  rock  phosphate 
and  bone-meal.  Sodium  nitrate  reduces  the  availability  of 
the  tri-calcium  phosphates,  while  the  ammonium  salts  increase 
their  availability.  Iron  and  aluminum  salts  decrease  avail- 
ability. The  influence  of  other  salts  has  not  been  so  well 
worked  out.  Prianischnikov,1  as  the  result  of  his  extended 
experiments  on  the  subject,  holds  that  salts  from  which  plants 
absorb  acid  radicals  in  larger  amounts  than  they  do  bases 
decrease  availability,  or  at  least  do  not  affect  it,  while  salts 
from  which  plants  absorb  the  bases  in  the  greater  quantity 
have  a  tendency  to  render  the  phosphate  more  available  be- 
cause of  the  hydrogen  ion  concentration. 

There  has  been  great  differences  of  opinion  among  investi- 
gators as  to  the  effect  of  the  decomposition  of  organic  matter 
on  the  availability  of  the  phosphorus  of  tri-calcium  phosphate. 
The  contention  that  the  availability  is  increased  probably 
originated  with  Stoklasa,1  whose  experiments  with  bone-meal 

1  Prianischnikov,  D.,  Vber  den  Einfluss  von  Kohlensauren  KalTc  auf  die 
Wirkung  von  Verschiedenen  Phosphaten:  Landw.  Vers.  Stat.,  Band  75, 
Seite  357-376,  1911. 

2  Stoklasa,  J.,  Duchacek,  F.,  and  Pitra,  J.,  trber  den  Einfluss  der  Bak- 
terien  auf  die  Knochenzersetzung ;  Centrlb.  f.  Bakt.,  II,  Band  6,  Seite 
526-535,  554-558,  1900. 


COMMERCIAL  FERTILIZER  MATERIALS      461 

indicate  that  the  availability  is  increased  by  decay.  A  large 
number  of  experiments  have  been  conducted  with  raw  rock 
phosphate  composted  with  stable  manure,  among  which  may 
be  mentioned  those  by  Hartwell  and  Pember  *  and  also  by 
Tottingham  and  Hoffman,2  who,  in  carefully  conducted  experi- 
ments, failed  to  find  that  the  availability  of  the  raw  phos- 
phate, as  indicated  by  chemical  methods,  was  increased  by 
fermentation  with  stable  manure.  Opposing  results  have  also 
been  obtained,  however,  and  the  evidence  is  somewhat  con- 
flicting. 

With  so  many  factors  active  in  varying  the  results,  espe- 
cially those  from  raw  rock  phosphate,  it  is  not  surprising  that 
satisfactory  field  data  where  acid  phosphate  and  raw  rock 
are  compared  are  difficult  to  obtain.  Thorne,3  after  a  critical 
review  of  the  field  experiments  where  acid  phosphate  and  raw 
rock  were  used,  comes  to  the  conclusion  that,  while  raw  rock 
phosphate  is  an  excellent  fertilizer,  acid  phosphate  is  gener- 
ally superior.  He  finds  that,  while  raw  rock  may  be  used 
with  profit  on  land  materially  deficient  in  phosphorus,  acid 
phosphate  has  generally  proven  to  be  the  more  effective  and 
the  more  economical  carrier  of  phosphoric  acid  for  crops. 

These  conclusions,  which  are  corroborated  by  other  in- 
vestigators,4 do  not  imply  that  raw  rock  phosphate  is  never 
equal  or  superior  to  acid  phosphate,  nor  that  raw  rock  does 
not  have  a  place  as  a  fertilizer  on  the  average  farm.     On  a 

1  Hartwell,  B.  L.,  and  Pember,  F.  E.,  The  Effect  of  Cow  Dung  on  the 
Availability  of  Rock  Phosphate;  K.  I.  Agr.  Exp.  Sta.,  Bui.  151,  1912. 

"Tottingham,  W.  E.,  and  Hoffman,  C,  The  Nature  of  the  Changes  in 
Solubility  and  Availability  of  Phosphorus  in  Fermenting  Mixtures;  Wis. 
Agr.  Exp.  Sta.,  Res.  Bui.  29,  1913. 

3  Thorne,  C.  E.,  Raw  Phosphate  Rock  as  a  Fertilizer;  Ohio  Agr.  Exp. 
Sta.,  Bui.  305,  1916. 

4  Wiancko,  A.  T.,  and  Conner,  S.  D.,  Acid  Phosphate  versus  Raw  Rock 
Phosphate  as  Fertilizer;  Purdue  Univ.  Agr.  Exp.  Sta.,  Bull.  187,  1916. 

Brooks,  W.  P.,  Phosphates  in  Massachusetts  Agriculture ;  Mass.  Agr. 
Exp.  Sta.,  Bull.  162,  1915. 

Waggaman,  W.  H.,  and  Wagner,  C.  R.,  Analysis  of  Experimental 
Work  with  Ground  Raw  Rock  Phosphate  as  a  Fertilizer;  U.  S.  Dept. 
Agr.,  Bui.  699,  1918. 


462        NATURE  AND  PROPERTIES  OF  SOILS 

soil  rich  in  organic  matter  it  may  be  added  to  advantage.  It 
is  especially  useful  in  reinforcing  farm  manure,  seemingly  be- 
ing about  as  effective  under  such  conditions  as  is  acid  phos- 
phate. Its  higher  phosphorus  content  and  lower  cost  a  ton 
gives  it  an  added  advantage.  The  figures  from  Ohio,1  cover- 
ing a  period  of  fourteen  years  in  a  rotation  of  maize,  wheat, 
and  hay  may  be  taken  as  evidence  regarding  these  points. 
The  manure,  reinforced  to  the  ton  with  40  pounds  of  acid 
phosphate  and  raw  rock  phosphate,  respectively,  was  applied 
to  the  corn  at  the  rate  of  eight  tons  to  the  acre. 

Table  XCVI 

A   COMPARISON    OF   ACID   PHOSPHATE   AND   RAW    ROCK    IN    EQUAL 
WEIGHTS  WHEN  ADDED  TO  THE  SOIL  WITH   MANURE. 


Manure 

Average  Annual  Increase  to  the 
Acre 

Maize 
14  Crops 

Wheat 
14  Crops 

Hay 

11  Crops 

With  raw  rock 

25.0  bu. 
30.6  bu. 

12.9  bu. 
15.1  bu. 

1578  lbs. 

With  acid  phosphate 

1853  lbs. 

FERTILIZERS  USED  FOR   THEIR  POTASSIUM 

The  production  of  potassium  fertilizers  is  largely  confined 
to  Germany,  where  there  are  extensive  beds  varying  from 
50  to  150  feet  in  thickness,  lying  under  an  area  extending 
from  the  Harz  Mountains  to  the  Elbe  River  and  known  as 
the  Stassfurt  deposits.  Large  deposits  of  crude  potash  salts 
occur  in  other  sections  of  Germany,  and  also  in  France. 
While  small  deposits  occur  in  other  parts  of  the  world  the 
French  and  German  mines  are  at  present  the  only  ones  of 
any  great  commercial  importance.  The  World  War  stimu- 
lated considerable  investigation  regarding  possible  sources  of 

1  Thome,  C.  E.,  et  al.,  Plans  and  Summary  Tables  of  the  Experiments 
at  the  Central  Farm;  Ohio.  Agr.  Exp.  Sta.,  Circ.  120,  p.  112,  1912. 


COMMERCIAL  FERTILIZER  MATERIALS      463 

potash,  especially  in  the  United  States.  Kelp,  saline  brines, 
deposits  in  old  lake  beds,  and  flue  dust  yielded  considerable 
potassium.  Most  of  these  sources,  however,  are  too  expensive 
to  compete  with  European  potash  in  normal  times. 

260.  Stassfurt  salts  and  their  refined  equivalents. — The 
Stassfurt  salts  contain  their  potassium  either  as  a  chloride 
or  as  a  sulfate.  The  chloride  has  the  advantage  of  being  more 
diffusible  in  the  soil,  but  in  most  respects  the  sulfate  is  pref- 
erable. Potassium  chloride  in  large  applications  has  an  in- 
jurious effect  on  certain  crops,  among  which  are  tobacco, 
sugar-beets,  and  potatoes.  On  cereals,  legumes,  and  grasses 
the  muriate  appears  to  have  no  injurious  effect. 

Kainit  is  the  most  common  of  the  crude  products  of  the 
Stassfurt  mines  and  is  imported  into  this  country  in  large 
amounts.  It  is  generally  a  greyish  vari-colored  salt,  soluble 
in  water  and  alkaline  to  litmus.  It  carries  from  12  to  14 
per  cent,  of  K20,  largely  as  potassium  sulfate.  Its  potash 
is  immediately  available  to  the  crop.  Below  is  a  typical 
analysis : 

K2S04    21.3  NaCl     34.6 

KC1 2.0  CaS04    1.7 

MgS04   14.5  Insoluble 8 

MgCl2     12.4  H20    12.7 

Silvinit  contains  its  potassium  both  as  a  chloride  and  as  a 
sulfate.  It  also  contains  sodium  and  magnesium  chlorides. 
Potash  constitutes  about  16  per  cent,  of  the  material.  Owing 
to  the  presence  of  chlorides,  it  has  the  same  effect  on  plants 
as  has  kainit.  There  are  a  number  of  other  Stassfurt  salts, 
consisting  of  mixtures  of  potassium,  sodium,  and  magnesium 
in  the  form  of  chlorides  and  sulfates.  They  are  not  so  widely 
used  for  fertilizers  as  are  those  mentioned  above. 

A  great  proportion  of  the  crude  salts  are  refined  for  ex- 
port purposes,  appearing  on  the  market  as  either  the  chloride 
or  the  sulfate.    They  usually  contain  from  48  to  50  per  cent. 


464        NATURE  AND  PROPERTIES  OF  SOILS 

of  potash.  The  chief  impurity  is  common  salt.  Some  of  the 
potash  salts  produced  in  this  country  carry  boron,  which  is 
extremely  toxic  to  plants.  Such  is  not  generally  true  of  the 
German  and  French  products. 

Potassium  chloride  and  potassium  sulfate  when  added  to 
the  soil  are  immediately  soluble,  being  held  in  the  soil  solu- 
tion or  absorbed  either  physically  or  chemically  by  the  col- 
loidal complexes.  Due  to  the  selective  absorption  of  the  soil 
for  the  potassium  ion  and  the  fact  that  plants  absorb  more  of 
this  ion  than  of  the  acid  radical,  an  acid  residue  tends  to  re- 
sult from  the  use  of  such  fertilizers.  Some  means,  such  as  the 
use  of  lime,  should  be  employed  to  counteract  this  tendency. 

261.  Other  sources  of  potash.1 — For  some  time  after  the 
use  of  fertilizers  became  an  important  farm  practice,  wood- 
ashes  were  the  source  of  most  of  the  potash.  They  also  con- 
tain a  considerable  quantity  of  lime  and  a  small  amount  of 
phosphorus.  The  product  known  as  unleached  wood-ashes 
contains  from  5  to  6  per  cent,  of  potash,  2  per  cent,  of  phos- 
phoric acid,  and  30  per  cent,  of  calcium  oxide.  Leached  wood- 
ashes  contain  about  1  per  cent,  of  potash,  iy2  per  cent,  of  phos- 
phoric acid,  and  from  28  to  29  per  cent,  of  lime  in  the  form 
of  the  hydroxide  and  carbonate.  Unleached  ashes  carry  the 
oxide,  hydroxide,  and  carbonate  forms  of  calcium.  Ashes 
contain  the  potassium  in  the  form  of  a  carbonate,  (K2C03), 
which  is  alkaline  in  its  reaction  and  in  large  amounts  may  be 
injurious  to  seeds.  Otherwise  this  form  of  potash  is  very  de- 
sirable, since  no  acid  residue  is  left  in  the  soil  by  its  use. 

1  Young,  G.  J.,  Potash  Salts  and  Other  Salines  in  the  Great  Basin; 
IT.  S.  Dept.  Agr.,  Bui.  61,  1914. 

Waggaman,  W.  H.,  and  Cullen,  J.  A.,  The  'Recovery  of  Potash  from 
Alunite;  U.  S.  Dept.  Agr.,  Bui.  415,  1916. 

Hirst,  C.  T.,  and  Carter,  E.  G.,  Some  Sources  of  Potassium;  Utah 
Agr.  Exp.  Sta.,  Circ.  22,  1916. 

Waggaman,  W.  H.,  The  Production  and  Fertiliser  Value  of  Citric- 
Soluble  Phosphoric  Acid  and  Potash;  U.  S.  Dent.  Agr.,  Bui.  143, 
1914.  t  &   >  t 

Eoss,  W.  H.,  et  al,  The  Recovery  of  Potash  as  a  By-Product  in  the 
Cement  Industry;  U.  S.  Dept.  Agr.,  Bui.  572,  1917. 


COMMERCIAL  FERTILIZER  MATERIALS      465 

Ashes  are  beneficial  to  acid  soils  through  the  action  of  both 
the  potassium  and  calcium  salts. 

Insoluble  forms  of  potassium,  existing  in  many  rocks 
usually  in  the  form  of  a  silicate,  are  not  regarded  as  having 
any  manurial  value.  Experiments  with  finely  ground  feld- 
spar have  been  conducted  by  a  number  of  investigators,  but 
have,  in  the  main,  offered  little  encouragement  for  the  suc- 
cessful use  of  this  material.  Leucite  and  alunite  have  given 
but  little  better  results.  An  insoluble  form  of  potassium  is 
not  recognized  as  of  value  when  a  fertilizer  is  rated  on  the 
basis  of  chemical  analysis. 

During  the  World  War,  since  the  German  importation  of 
potash  salts  ceased,  potassium  was  sought  commercially  from 
a  number  of  sources  in  this  country.  Alunite,  a  hydrous  sul- 
fate of  aluminum  and  potassium,  has  been  experimented  with 
to  some  extent  as  have  also  the  green-sand  marls  which  carry 
glauconite.  In  a  number  of  cases  the  recovery  of  potash 
from  flue  dust  has  proven  commercially  profitable.  It  is  esti- 
mated that  87,000  tons  of  potash  are  lost  yearly  from  cement 
kilns  alone  in  the  United  States  and  Canada.  During  the  war 
considerable  progress  was  made  in  harvesting  and  drying  the 
kelp  which  grows  off  the  coast  of  southern  California.  The 
kelp  was  later  extracted  for  its  potash.  This  source'  of  potas- 
sium is  rather  expensive,  however,  when  brought  into  com- 
petition with  European  products. 

Perhaps  the  most  reliable  sources  of  domestic  potash  are 
the  brines  of  certain  alkali  lakes  of  western  United  States  and 
from  the  deposits  in  old  lake  beds  in  the  same  region.1  The 
exploitation  of  such  sources  will,  of  course,  depend  upon  the 
price  at  which  German  potash  can  be  laid  down  in  this 
country. 

1  Such  salts  unless  properly  prepared  are  likely  to  contain  borax 
which  is  usually  toxic  when  applied  at  a  greater  rate  than  five  pounds 
to  the  acre,  the  influence  being  more  intense  at  low  soil  moisture. 

Neller,  J.  K.,  and  Morse,  W.  J.,  Effects  upon  the  Growth  of  Potatoes, 
Com  and  Beans,  Resulting  from  the  Addition  of  Borax  to  the  Fertilizer 
used;  Soil  Sci.,  Vol.  XII,  No.  2,  pp.  79-105,  1921. 


466        NATURE  AND  PROPERTIES  OF  SOILS 

SULFUR  AND  SULFATES  AS  FERTILIZERS  * 

The  use  of  these  substances  as  a  means  of  increasing  plant 
growth  when  applied  to  soils  has  recently  received  much  at- 
tention. While  sulfates  have  been  used  for  centuries  as  a 
soil  amendment,  it  is  only  within  the  last  few  years  that  sulfur 
itself  has  been  applied  to  soil.  The  question  of  the  effect  of 
the  latter  has  received  considerable  study,  not  only  in  France 
and  Germany  but  in  this  country  as  well.  The  influence  of 
both  sulfur  and  sulfates  may  be  a  direct  nutrient  relationship 
or  the  action  may  be  that  of  a  soil  amendment.  Only  in  case 
the  former  influence  occurs  could  these  materials  be  rated  as 
fertilizers. 

262.     The  use  of  free  sulfur.— Boullanger 2  in  1912  added 

1  Another  group  of  fertilizers  may  be  mentioned — the  so-called  catalytic 
fertilizers.  Such  materials  are  supposed  to  aid  plant  growth  by  accelerat- 
ing natural  soil  processes.  The  catalytic  action  of  any  material  is  very 
difficult  to  establish  when  it  is  added  to  the  soil,  since  the  soil  itself 
carries  many  substances  of  a  catalytic  nature.  Manganese  has  been  most 
seriously  considered  as  a  catalytic  fertilizer. 

Konig,  J.,  Hasenbaumer,  J.,  and  Coppenrath,  E.,  Einige  Neue  Eigen- 
schaften  des  Acker o odens ;  Landw.  Vers.  Stat.,  Band  63,  Seite  471-478, 
1905-1906. 

May,  D.  W.,  and  Gile,  P.  L.,  The  Catalase  of  Soils;  Porto  Eico  Agr. 
Exp.  Sta.,  Circ.  9,  1909. 

Sullivan,  M.  X.,  and  Reid,  F.  R.,  Studies  in  Soil  Catalysis;  U.  S. 
Dept.  Agr.,  Bur.  Soils,  Bui.  86,  1912. 

Konig,  J.,  Hasenbaumer,  J.,  and  Coppenrath,  E.,  Beziehungen  zwischen 
den  Eigenschaften  des  Bodens  und  der  Nahrstoffaufnahme  durch  die 
pflanzen;  Landw.  Vers.  Stat.,  Band  66,  Seite  401-461,  1907. 

Kelly,  M.  P.,  The  Influence  of  Manganese  on  the  Growth  of  Pine- 
apples; Jour.  Ind.  and  Eng.  Chem.,  Vol.  I,  p.  533,  1909. 

Sullivan,  M.  X.,  and  Robinson,  W.  O.,  Manganese  as  a  Fertilizer; 
U.  S.  Dept.  Agr.,  Bur.  Soils,  Circ.  75,  1912. 

Skinner,  J.  J.,  and  Sullivan,  M.  X.,  The  Action  of  Manganese  in  Soils: 
U.  S.  Dept.  Agr.,  Bui.  42,  1914. 

Skinner,  J.  J.,  and  Reid,  F.  R.,  The  Action  of  Manganese  Under  Acid 
and  Neutral  Soil  Conditions;  U.  S.  Dept.  Agr.,  Bui.  441,  1916. 

Bertrand,  G.,  The  Action  of  Chemical  Infinitesimals  in  Agriculture; 
Address  before  8th  Inter.  Cong.  App.  Chem.,  New  York,  1912. 

Ross,  W.  H.,  The  Use  of  Badioactive  Substances  as  Fertilizers;  U.  S. 
Dept.  Agr.,  Bui.  149,  1914. 

Hopkins,  C.  G.,  and  Sachs,  W.  H.,  Badium  as  a  Fertilizer;  111.  Agr. 
Exp.  Sta.,  Bui.  177,  1915. 

2  Boullanger,  E.,  Action  du  soufre  en  fleur  sur  la  vegetation;  Compt. 
Rend.  Acad.  Sci.  Paris,  T.  154,  pp.  369-370,  1912. 


COMMERCIAL  FERTILIZER  MATERIALS      467 

flowers  of  sulfur  to  a  soil  at  the  rate  of  23  parts  per  million 
of  soil.  He  obtained  increased  growth  in  all  treated  soils  on 
which  carrots,  beans,  celery,  lettuce,  sorrel,  chicory,  potatoes, 
onions,  and  spinach  were  grown,  the  weights  of  the  crops  on 
the  treated  soil  being  from  10  to  40  per  cent,  greater  than  those 
on  the  untreated  soil.  On  soils  that  had  been  sterilized  before 
applying  sulfur,  the  effect  was  less  marked,  from  which  he 
concludes  that  the  beneficial  effects  were  due  to  the  influence 
of  the  sulfur  on  the  micro-organisms  of  the  soil.  There  may 
be  some  question,  however,  whether  this  conclusion  is  justi- 
fiable. Sulfur  was  found  by  Boullanger  and  Dugardin  *  to 
favor  ammonification  in  soils.  Beneficial  effects  from  the  use 
of  free  sulfur  have  also  been  obtained  by  Demelon,2  and  by 
Bernhard,3  while  von  Feilitzen  4  found  it  to  be  ineffective  as 
a  fertilizer. 

In  this  country,  Shedd  5  of  Kentucky  obtained  increases  in 
tobacco  yield  with  sulfur.  Perhaps  the  most  marked  results 
with  sulfur  are  reported  by  Reimer  and  Tartar  6  from  Oregon. 
Alfalfa  and  clover  yields  were  increased  from  50  to  100  per 
cent. 

That  free  sulfur  may,  under  certain  conditions,  exert  a  ben- 
eficial influence  on  plant  growth  must  be  conceded,  but  that 
the  action  is  a  direct  nutritive  one  remains  to  be  proven. 
Free  sulfur  is  insoluble  and  cannot  be  absorbed  as  such  by 
plants.  It  readily  undergoes  oxidation,  however,  producing 
the  sulfate,  as  already  explained  under  sulfofication.    As  such 

1  Boullanger,  E.,  and  Dugardin,  M.,  Mecanisme  de  V action  fertilisante 
du  soufre;  Compt.  Eend.  Acad.  Sci.  Paris,  T.  155,  pp.  327-329,  1912. 

2  Demelon,  A.,  Sur  V action  fertilisante  du  soufre;  Compt.  Eend.  Acad. 
Sci.  Paris,  T.  154,  pp.  524-526,  1912. 

3  Bernhard,  A.,  Versuche  iiber  dis  Wirkung  des  Schwefels  als  Dung  im 
Jahre  1911;  Deutsche  Landw.  Presse.,  Band  39,  S.  275,  1912. 

4  von  Feilitzen,  H.}  tiber  die  Verwendung  der  Schwefelblute  zur  Be- 
Tcampfung  des  Kartoffelschorfes  und  als  indirktes  Dungemittel;  Fuhling's 
Landw.  Zeit.,  Band  62,  Seite  7,  1913. 

6  Shedd,  O.  M.,  The  Belation  of  Sulfur  to  Soil  Fertility :  Ky.  Agr.  Exp. 
Sta.,  Bui.  188,  1914. 

"Eeimer,  F.  C,  and  Tartar,  H. 
in  Southern  Oregon;  Ore.  Agr.  Exp.  Sta..  Bui,  163f  1919. 


468        NATURE  AND  PROPERTIES  OF  SOILS 

a  reaction  tends  to  encourage  soil  acidity,  injurious  influ- 
ences may  easily  occur  on  soils  already  acid  or  possessing  only 
small  quantities  of  active  calcium  and  magnesium.  If  sulfur 
functions  as  a  f ertilizer, LJtjs^y_a_change  to  the  sulfate,  in 
which  form  it  is  absorbed  by  plants. 

263.  The  use  of  sulfate  sulfur. — The  experimental  evi- 
dence regarding  the  direct  fertilizer  influence  of  sulfate  sulfur 
is  much  more  difficult  to  interpret  than  that  regarding  flowers 
of  sulfur.  Gypsum  has  been  applied  to  soils  for  centuries 
and  marked  influences  on  crop  growth  are  of  common  observa- 
tion. Whether  this  stimulation  is  due  to  the  sulfate  or  to  the 
base  which  accompanies  it  cannot  be  determined.  Even  if  the 
sulfate  influence  could  definitely  be  proved,  there  would  still 
remain  the  question  as  to  whether  the  action  was  direct  or 
indirect. 

264.  Relation  of  sulfur  to  soil  fertility. — The  possible 
deficiency  of  sulfur  in  arable  soils  was  first  established  by 
Hart  and  Peterson.1    They  point  out  that  crops  remove  more 

Table  XCVII 

POUNDS  SULFUR  TRIOXIDE  AND  PHOSPHORUS  PENTOXIDE 
REMOVED  TO  THE  ACRE  BY  AVERAGE  CROPS. 


Crop  and  Yield  to  the  Acre 


Pounds  to  the  Acre 


SO, 

P,05 

15.7 

21.1 

14.3 

20.7 

19.7 

19.7 

12.0 

18.0 

64.8 

39.9 

92.2 

33.1 

98.0 

61.0 

11.5 

21.5 

11.3 

12.3 

Wheat  (30  bu.) 

Barley  (40  bu.) \ 

Oats  (45  bu.) 

Corn  (30  bu.) 

Alfalfa  (9000  lbs.  air  dry) ... . 
Turnips  (4657  lbs.  air  dry) 
Cabbage  (4800  lbs.  air  dry) ... . 
Potatoes  (3360  lbs.  air  dry) 
Meadow  hay  (2822  lbs.  air  dry) 


1  Hart,  E.  B.,  and  Peterson,  W.  H.,  Sulfur  'Requirements  of  Farm  Crops 
in  Relation  to  the  Soil  and  Air  Supply;  Wis.  Agr.  Exn.  Sta.,  Res.  Bui. 
14,1911.  8  '  ' 


COMMERCIAL  FERTILIZER  MATERIALS       469 

sulfur  from  the  soil  than  is  indicated  by  the  earlier  analyses 
of  plant  ash,  since  considerable  sulfur  was  lost  by  volatization 
in  the  former  determination.  On  the  basis  of  their  own 
methods,  they  present  the  data  given  as  to  the  removal  of 
sulfur  trioxide  and  phosphoric  acid  from  the  soil  by  average 
crops.     (See  Table  XCVII,  page  468.) 

It  is  to  be  noted  that  the  amount  of  sulfur  removed  by  crops 
is  generally  about  equal  to  and  in  some  cases  much  in  excess 
of  the  phosphoric  acid  taken  from  the  soil.  The  fact  that 
soils  are  generally  as  low  in  sulfur  as  in  phosphoric  acid  lends 
weight  to  the  argument,  that  if  the  latter  is  a  limiting  factor 
in  productivity  the  former  should  be  also. 

To  ascertain  whether  the  supply  of  sulfur  in  the  soil  is 
really  depleted  by  cropping,  Hart  and  Peterson  made  parallel 
determinations  of  sulfur  in  five  virgin  soils  and  in  five  soils  of 
the  same  respective  types  that  had  been  cropped  for  sixty 
years.  In  each  type  the  cropped  soil  contained  less  sulfur 
than  the  virgin  soil,  the  average  for  the  former  being  .053 
per  cent.  S03  and  for  the  latter  .085  per  cent.  S03. 

Considerable  sulfur  is  added  to  the_soil  every_year  in  the 
rain-water,  largely  in  the  sulfate  form,  although  near  cities 
appreciable  amounts  of  hydrogen  sulfide  and  sulfur  di-oxide 
are  formed.  The  amount  of  such  sulfur  is  variable.  Miller,1 
at  the  Rothamsted  Experiment  Station,  reports  17.4  pounds 
of  S03  to  the  acre,  while  Crowther  and  Ruston  2  near  Leeds, 
England,  found  161  pounds  of  S03  to  the  acre.  Peck 3  found 
the  addition  of  S03  to  be  at  the  rate  of  1  pound  to  the  acre  a 
month   at   Mt.   Vernon,   Iowa,   while    Trieschmann,4   over   a 

1  Miller,  N.  H.  J.,  The  Amount  of  Nitrogen,  as  Ammonia  and  as 
Nitric  Acid,  and  of  Chlorine  in  the  Bain-Water  Collected  at  'Rotham- 
sted; Jour.  Agr.  Sci.,  Vol.  I,  pp.  280-303,  1905. 

8  Crowther,  C,  and  Euston,  A.  C,  The  Nature,  Distribution  and 
\Effect  Upon  Vegetation  of  Atmospheric  Impurities  In  and  Near  an 
Industrial  Town;  Jour.  Agr.  Sci.,  Vol.  4,  pp.  25-55,  1911. 

8  Peck,  E.  L.,  Nitrogen,  Chlorine  and  Sulfates  in  Bain  and  Snow; 
Chem.  News.,  Vol.  116,  p.  283,  1917. 

4  Trieschmann,  J.  E.,  Nitrogen  and  other  Compounds  in  Bain  and 
Snow;  Chem.  News,  Vol.  119,  p.  49,  1919. 


470        NATURE  AND  PROPERTIES  OP  SOILS 

different  period  at  the  same  place,  determined  the  addition  to 
be  less  than  .2  pound  a  month.  Stewart,1  at  the  University 
of  Illinois,  reports  the  addition  of  sulfur  as  S03  over  a  period 
of  seven  years  as  amounting  to  9.4  pounds  of  S03  monthly  to 
the  acre  or  113  pounds  yearly. 

The  loss  of  sulfur  expressed  as  S03  from  the  Cornell  lysi- 
meters,2  due  to  cropping  and  drainage  combined,  amounted, 
over  a  period  of  ten  years,  to  149.5  pounds  from  an  acre 
yearly  from  the  rotation  tanks.  The  addition  of  sulfur  in  the 
rain-water  at  Ithaca  amounts  to  about  65.4  pounds  of  SOs 
each  year.  It  is,  therefore,  safe  to  assume  that  rain-water  will 
not  replace  the  sulfur  removed  by  normal  cropping  and 
leaching.  It  must  be  remembered,  however,  that  in  rational 
soil  management,  sulfur  is  returned  to  the  soil  in  green- 
manures,  crop  residues  and  farm  manures.  Commercial  fer- 
tilizers are  now  very  commonly  used,  especially  acid  phos- 
phate, which  is  about  one-half  gypsum.  At  the  Ohio  Experi- 
ment Station,3  plats  treated  with  sulfate  bearing  fertilizers 
were  found  over  a  period  of  years  to  contain  considerably 
more  sulfur  than  soils  not  so  fertilized  but  cropped  in  a 
similar  manner. 

In  the  light  of  such  data  it  seems  that  the  sulfur  problem 
is  not  comparable  with  or  as  serious  as  the  phosphorus  prob- 
lem of  soil  fertility.  By  the  careful  utilization  of  the  normal 
residues  produced  on  the  farm  there  seems  little  reason  for 
sulfur  being  a  limiting  factor  in  soil  productivity,  especially 
if  fertilizers  carrying  sulfur  are  used  in  connection  with  a 
rational  system  of  soil  management. 

1  Stewart,  R.,  Sulfur  in  Relation  to  Soil  Fertility;  111.  Agr.  Exp.  Sta., 
Bui.  227,  1920. 

'Complete  data  on  these  lysimeters  will  be  found  in  par.  163  of  this 
text. 

3  Ames,  J.  W.,  and  Boltz,  G.  E.,  Sulfur  in  Relation  to  Soils  and  Crops; 
Ohio  Agr.  Exp.  Sta.,  Bui.  292,  1916. 


CHAPTER  XXIII 
THE  PRINCIPLES  OF  FERTILIZER  PRACTICE  1 

The  use  of  commercial  fertilizers  has  increased  so  rapidly 
within  the  last  decade  that  specific  knowledge  is  needed  re- 
garding the  various  materials  offered  for  sale  in  order  that 
the  most  economical  results  may  be  attained.  The  greater 
the  general  knowledge,  both  practical  and  theoretical,  that  a 
person  possesses  as  to  the  effects  of  the  different  nutrient  con- 
stituents on  plant  growth,  the  more  rational  will  be  the  fer- 
tilizer use.  Fertilizer  inspection  and  control,  principles  of 
buying  and  home-mixing,  methods  of  application,  mixtures  for 
special  crops,  are  a  few  of  the  many  phases  of  economical 
fertilizer  practice.  The  final  and  vital  consideration  is  re- 
garding the  financial  return  from  fertilizer  application.  A 
fertilizer  should  always  pay. 

As  all  fertilizers  exert,  either  directly  or  indirectly,  a  resid- 
ual effect,  the  problem  necessarily  broadens  into  a  study  of 
the  systems  of  applying  them  to  a  series  of  crops  or  to  a  rota- 
tion, rather  than  a  study  of  the  effects  of  one  particular  fer- 
tilizer application  on  one  particular  crop. 

265.  Influence  of  nitrogen  on  plant  growth.2 — Of  the 
three   elements  carried  in   an   ordinary   complete   fertilizer, 

1Hall,  A.  D.,  Fertilizers  and  Manures;  New  York,  1921. 
Halligan,  J.  E.,  Soil  Fertility  and  Fertilizers;  Easton,  Pa.,  1912. 
Van  Slyke,  L.  L.,  Fertilizers  and  Crops;  New  York,  1912. 
Fraps,  G.  S.,  Principles  of  Agricultural  Chemistry;  Easton,  Pa.,  1913. 
"Discussions  of  the  effects  of  the  various  elements  on  plants  may  be 
found  as  follows:     Eussell,  E.  J.,  Soil  Conditions  and  Plant  Growth, 
Chapter  II,  pp.  19-50;  London,  1912.    Also,  Hall,  A.  D.,  Fertilizers  and 
Manures,  Chapters  III,  IV  and  VI;  New  York,  1921. 

471 


472        NATURE  AND  PROPERTIES  OF  SOILS 

nitrogen  1  seems  to  have  the  quickest  and  most  pronounced 
effect,  not  only  when  present  in  excess  of  other  constituents, 
but  also  when  moderately  used.  It  tends  primarily  to  encour- 
age above  ground  vegetative  growth,  and  to  impart  to  the 
leaves  a  deep  green  color,  a  lack  of  which  is  usually  due  to 
insufficient  nitrogen.  It  tends  in  cereals  to  increase  the 
plumpness  of  the  grain,  and  with  all  plants  it  is  a  regulator 
in  that  it  governs  to  a  certain  extent  the  utilization  of  potash 
and  phosphoric  acid.  Its  application  tends  to  produce  succu- 
lence, a  quality  particularly  desirable  in  certain  crops.  In  its 
general  effects  it  is  very  similar  to  moisture,  especially  when 
supplied  in  excessive  quantities. 

The  peculiarity  of  nitrogen  lies  not  only  in  its  absolute  ne- 
cessity for  plant  growth,  its  stimulation  of  the  vegetative 
parts,  and  its  close  relationship  to  the  general  tone  and  vigor 
of  the  crop,  but  also  in  the  fact  that  it  was  not  one  of  the 
original  elements  of  the  earth's  crust.  During  the  formation 
of  the  soil  it  slowly  and  gradually  became  present,  brought 
down  by  rains  and  fixed  naturally  in  the  soil  through  the 
agency  of  bacterial  action.  Now  it  exists  in  complex  nitrog- 
enous compounds  of  the  more  or  less  decayed  organic  matter, 
and  becomes  available  to  plants  largely  through  bacterial 
activity. 

It  may  be  stated  with  certainty  that  one  of  the  possible 
limiting  factors  to  crop  growth  is  a  lack  of  water-soluble  nitro- 
gen at  critical  periods  in  amounts  necessary  for  normal  devel- 
opment. Since  soluble  nitrogen  may  be  very  readily  lost 
from  the  soil  by  leaching,  the  problem  of  proper  plant  nutri- 
tion becomes  a  serious  one.  Not  only  must  the  farmer  be  able 
so  to  regulate  the  addition  of  nitrogen  in  fertilizers  as  to  obtain 
the  highest  efficiency,  but  he  must  understand  the  control  and 

1  For  a  discussion  of  nitrogen  in  relation  to  crop  yield,  see  Hunt,  T.  F., 
The  Importance  of  Nitrogen  in  the  Growth  of  Plants:  Cornell  Agr. 
Exp.  Sta.3  Bui.  247,  1907. 


THE  PRINCIPLES  OF  FERTILIZER  PRACTICE    473 

encouragement  of  the  natural  fixation  as  well.  Due  to  the 
practical  possibility  of  keeping  up  the  nitrogen  supply  of  the 
soil  by  the  proper  use  of  farm  manure,  crop  residues,  green- 
manures,  and  the  utilization  of  legumes  in  the  rotation,  the 
quantity  of  nitrogen  purchased  in  commercial  fertilizers 
should  be  as  small  as  possible  if  its  use  is  to  be  profitable. 
When  so  purchased  it  should  function  more  or  less  as  a  crop 
starter  rather  than  as  a  source  of  any  large  amount  of  the 
plants'  supply  of  nutrient.  The  emphasis  placed  on  all  phases 
of  the  nitrogen  problem  serves  to  reveal  its  great  importance 
in  fertility  practices. 

Because  of  the  immediately  visible  effect  from  the  applica- 
tion of  soluble  nitrogen,  the  average  farmer  is  prone  to  ascribe 
too  much  importance  to  its  influence  in  proper  crop  develop- 
ment. This  attitude  is  unfortunate,  since  nitrogen  is  the 
highest  priced  constituent  of  ordinary  fertilizers  and  should 
usually  be  purchased  to  a  less  extent  than  potash  and  espe- 
cially than  phosphoric  acid.  Moreover,  of  the  three  common 
fertilizer  elements,  it  is  the  only  one  which,  added  in  excess, 
will  result  in  harmful  after-effects  on  the  crop.  These  pos- 
sible and  important  detrimental  effects  of  nitrogen  may  be 
listed  as  follows : 

1.  It  may  delay  maturity  by  encouraging  vegetative 
growth.  This  oftentimes  endangers  the  crop  to  frost,  or  may 
cause  trees  to  winter  badly. 

2.  It  may  weaken  the  straw  and  cause  lodging  in  grain. 
This  is  due  to  an  extreme  lengthening  of  the  internodes,  and 
as  the  head  fills  the  stem  is  no  longer  able  to  support  the  in- 
creased weight. 

3.  It  may  lower  quality.  This  is  especially  noticeable  in 
certain  grains  and  fruits,  as  barley  and  peaches.  The  ship- 
ping qualities  of  fruits  and  vegetables  are  also  impaired. 

4.  It  may  decrease  resistance  to  disease.  This  is  probably 
due  to  a  change  in  the  physiological  resistance  within  the 


474        NATURE  AND  PROPERTIES  OF  SOILS 

plant,  and  also  to  a  thinning  of  the  cell-wall,  allowing  a  more 
ready  infection  from  without. 

While  certain  plants,  as  the  grasses,  lettuce,  radishes,  and 
the  like,  depend  for  their  usefulness  on  plenty  of  nitrogen,  it 
is  generally  better  to  limit  the  amount  of  nitrogen  for  the 
average  crop  so  that  growth  may  be  normal.  This  results  in 
a  better  utilization  of  the  nitrogen  and  in  a  marked  reduction 
of  the  fertilizer  cost  for  a  unit  of  crop  growth.  This  is  a 
vital  factor  in  all  fertilizer  practice,  and  shows  immediately 
whether  nitrogen  fertilization  is  or  is  not  an  economic  success. 

266.  Influence  of  phosphorus  on  plant  growth. — It  is 
difficult  to  determine  exactly  the  functions  of  phosphoric  acid 
in  the  economy  of  even  the  simplest  plants.  Neither  cell  divi- 
sion nor  the  formation  of  fat  and  albumen  go  on  to  a  suffi- 
cient extent  without  it.  Starch  may  be  produced  when  it  is 
lacking,  but  will  not  change  to  sugar.  As  grain  does  not  form 
without  its  presence,  it  very  probably  is  concerned  in  the  pro- 
duction of  nucleoproteid  materials.  Its  close  relationship  to 
cell  division  may  account  for  its  presence  in  seeds  in  compara- 
tively large  amounts. 

Phosphoric  acid  hastensjthe  maturity  of  the  crop  by  its 
ripemn^Jnfluences.  This  effect  is  especially  valuable  in  wet 
years  and  in  cold  climates  where  the  season  is  short.  The  use 
of  acid  phosphate  is  being  advocated  in  the  Middle  West,  espe- 
cially for  maize,  as  an  insurance  against  frost-injury  and  a 
means  of  avoiding  soft  corn.  Phosphoric  acid  also  encourages 
root  development,  especially  of  lateral  and  fibrous  rootlets. 
This  renders  it  valuable  in  such  soils  as  do  not  encourage  root 
extension  and  to  such  crops  as  naturally  have  a  restricted  root 
development.  Phosphoric  acid  is  especially  valuable  for  fall- 
sown  crops,  such  as  wheat.  A  sturdy  root  growth  is  developed 
which  tends  to  prevent  winter  injury  and  prepares  the  plant 
for  a  rapid  spring  development. 

Phosphoric  acid  decreases  the  ratio  of  straw  to  grain  in 
cereals.     It  also  strengthens  the  straw,  thus  decreasing  the 


THE  PRINCIPLES  OF  FERTILIZER  PRACTICE    475 

tendency  to  lodge,  which  is  likely  to  occur  especially  with 
oats  if  too  much  available  nitrogen  is  present.  In  certain 
cases,  phosphoric  acid  decidedly  improves  the  quality  of  the 
crop.  This  has  been  recognized  in  the  handling  of  pastures 
in  England  and  France.  The  effect  on  vegetables  is  also 
marked.  Phosphorus  is  also  known  to  increase  the  resistance 
of  some  plants  to  disease,  due  possibly  to  a  more  normal  cell 
development.  In  this  respect  phosphoric  acid  counteracts  the 
influence  of  a  heavy  nitrogen  ration. 

Excessive  quantities  of  phosphoric  acid  ordinarily  have  no 
bad  effect,  as  phosphorus  does  not  stimulate  any  part  unduly, 
nor  does  it  lead  to  a  development  which  is  detrimental.  The 
lack  of  phosphoric  acid  is  not  apparent  in  the  color  of  the 
plants  as  in  the  case  of  nitrogen,  and  as  a  consequence  phos- 
phoric acid  starvation  may  occur  without  any  suspicion  there- 
of being  entertained  by  the  farmer. 

One  of  the  most  important  phases  to  be  noted  from  this 
comparison  of  the  effects  of  nitrogen  and  phosphorus  is  the 
balancing  powers  of  the  latter  on  the  unfavorable  influences 
generated  by  the  presence  of  an  undue  quantity  of  the  former. 
The  possible  detrimental  effects  of  too  much  nitrogen  have 
already  been  noted.  This  relationship  between  the  phosphorus 
and  nitrogen  in  plant  nutrition  is  very  important  in  fertilizer 
practice,  since  normal  fertilizer  stimulation  generally  results 
in  the  most  economical  gains. 

267.  Effects  of  potassium  on  plant  growth. — The  pres- 
ence of  plenty  of  available  potash  in  the  soil  has  much  to  do 
with  the  general  tone  and  vigor  of  the  plant.  By  increasing 
resistance  to  certain  diseases  it  tends  to  counteract  the  ill 
effects  of  too  much  nitrogen,  while  in  delaying  maturity  it 
works  against  the  ripening  influences  of  phosphoric  acid.  In 
a  general  way,  it  exerts  a  balancing  effect  on  both  nitrogen 
and  phosphate  fertilizer  materials,  and  consequently  is  espe- 
cially important  in  a  mixed  fertilizer,  if  the  potash  of  the 
soil  is  lacking  or  unavailable. 


476        NATURE  AND  PROPERTIES  OF  SOILS 

Potash  is  essential  to  starch  formation,  either  in  photo- 
synthesis or  in  translocation,  and  is  necessary  in  the  develop- 
ment of  chlorophyll.  It  is  important  to  cereals  in  grain  for- 
mation, giving  plump  heavy  kernels.  As  with  phosphorus,  it 
may  be  present  in  large  quantities  in  the  soil  and  yet  exert 
no  harmful  effect  on  the  crop.  While  potassium  and  sodium 
are  similar  in  a  chemical  way,  sodium  cannot  take  the  place 
of  potash  in  plant  nutrition.  Where  there  is  an  insufficiency 
of  potash,  however,  sodium  seems  in  some  way,  either  directly 
or  indirectly,  to  be  useful.1 

268.  The  element  in  the  "minimum." — In  connection 
with  the  obvious  importance  of  utilizing,  for  any  particular 
soil  and  crop,  a  fertilizer  well  balanced  as  to  the  three  primary 
elements,  two  queries  naturally  arise.  These  are:  (1)  What 
are  the  proper  proportions  of  nitrogen,  phosphoric  acid,  and 
potash  to  apply  under  given  conditions?  (2)  What  would 
be  the  effect  if  any  one  of  these  should  not  be  present  in  suffi- 
cient quantity  as  to  make  it  equal  in  function  to  the  others? 

The  first  query  cannot  be  disposed  of  until  the  question  of 
fertilizer  mixtures  has  been  considered.  The  second,  how- 
ever, is  not  affected  by  so  many  factors,  and  is  more  clearly 
a  question  of  the  function  of  the  elements  concerned  and  is 
logically  discussed  at  this  point. 

Any  element  that  exists  in  relatively  small  amounts  as  com- 
pared with  the  other  important  nutrient  constituents  natur- 
ally becomes  the  controlling  factor  in  plant  development. 
Any  reoluction  or  increase  in  this  element  will  cause  a_corre- 
sponding  reduction  or  increase  in  the  crop  yield.  This  ele- 
ment, then,  is  said  to  be  "in  the  minimum."  In  fertilizer 
practice,  ideal  conditions  would  exist  if  no  constituent  func- 
tioned as  a  decided  minimum  and  the  entire  influence  of  each 
single  element  was  fully  utilized.  In  other  words,  the  fertil- 
izer would  be  balanced  as  to  its  relationship  to  normal  plant 

JHartwell,  B.  L.,  and  Damon,  S.  C,  The  Value  of  Sodium  when 
Potassium  is  Insufficient;  E.  I.  Agr.  Exp.  Sta.,  Bui.  177,  1919. 


THE  PRINCIPLES  OF  FERTILIZER  PRACTICE    477 

growth.  That  such  a  condition  is  more  or  less  ideal  and  is 
seldom  realized  is  obvious,  from  the  fact  that  the  various  fer- 
tilizer carriers  undergo  more  or  less  radical  changes  after 
being  applied  to  the  soil.  The  composition  of  the  soil  itself 
is  also  a  disturbing  factor.  Nevertheless,  the  nearer  an  ap- 
proach can  be  made  to  such  conditions,  the  greater  will  be  the 
economy  in  fertilizer  practice. 

Numerous  persons  have  investigated  the  question  as  to  what 
effect  an  increase  of  an  element  in  the  minimum  may  have  on 
crop  yield,  and  various  ideas  have  been  advanced  to  explain 
the  effect.  The  idea  of  a  definite  law  governing  the  increase 
of  plant  growth  according  as  the  element  in  the  minimum  is 
increased,  was  first  suggested  by  Liebig.  Wagner x  later 
stated  definitely  that  up  to  a  certain  point  the  increase  yield 
was  proportional  to  the  increase  in  the  application.  This, 
however,  evidently  cannot  apply  except  over  a  very  limited 
field,  since  it  is  a  matter  of  common  observation  that  increased 
crop  yield  becomes  lower  as  the  lacking  element  is  continu- 
ously supplied. 

Mitscherlich  2  has  formulated  a  law  which  is  a  logarithmic, 
rather  than  a  direct,  function  of  the  increase  in  the  element 
occupying  the  position  of  the  minimum.  Mitscherlich 's  law 
may  be  stated  concisely  as  follows :  the  increased  growth  pro- 
duced by  a  unit  increase  of  the  element  in  the  minimum  is 
proportional  to  the  decrement  from  the  maximum.  In  other 
words,  the  increase  is  proportional  to  the  difference  between 
the  actual  yield  and  the  possible  yield  at  which  the  element 
ceases  to  be  a  limiting  factor. 

Mitscherlich  has  proposed  a  definite  formula  for  such  a 

1  Wagner,  H.,  Beitrage  zur  Dungerlehre;  Landw.  Jahr.,  Band  12, 
Seite  691  ff.,  1883. 

2  Mitscherlich,  E.  A.,  Das  Gesetz  des  Minimums  und  das  Gesetz  des 
Abnehmen  den  Bodenertrages;  Landw.  Jahr.,  Band  38,  Seite  537-552, 
1909. 

Also,  Ein  Beitrage  zur  Erforschung  der  Ausnutzung  des  im  Minimum 
Vorhandenen  Nahrstoffes  durch  die  Pflanze;  Landw.  Jahr.,  Band  39, 
Seite  133-156,  1910. 


478        NATURE  AND  PROPERTIES  OF  SOILS 

growth  curve.1  This  formula  has  been  questioned  by  several 
investigators,2  who  have  shown  that  a  number  of  conditions, 
such  as  light,  heat,  and  moisture,  tend  to  disturb  the  applica- 
tion of  such  a  law.  The  fact  that  crop  yield  is  the  summation 
of  so  many  varying  factors  seems  to  argue  in  favor  of  no  hard 
and  fast  rule  regarding  the  increased  growth  due  to  the  added 
increments  of  an  element  in  the  minimum.  It  is  enough,  in 
the  practical  utilization  of  fertilizers,  to  remember  that  in 
order  to  obtain  the  best  results  from  fertilizers  a  mixture 
should  be  used  that  is  approximately  balanced  so  far  as  the 
effects  of  the  nutrients  are  concerned,  the  crop  as  well  as  the 
chemical  constitution  of  the  soil  being  considered. 

269.  Fertilizer  brands. — In  an  attempt  to  meet  the  de- 
mands for  well-balanced  fertilizers  suited  to  various  crops  and 
soils,  manufacturers  have  placed  on  the  market  a  large  num- 
ber of  brands  of  materials  containing  usually  at  least  two  of 
the  important  nutrient  elements,  and  nearly  always  the  three ; 
the  former  being  designated  as  incomplete  fertilizers,  while 
the  latter  are  spoken  of  as  complete.  These  various  brands 
usually  have  a  significant  name,3  which  frequently  implies  the 
usefulness  of  the  material  for  some  special  crop  growing  on  a 
particular  soil.  Oftener,  however,  the  brand  name  bears  no 
relation  either  to  crop  or  soil.  The  name  should  always  be 
ignored  in  fertilizer  purchase,  the  availability  and  composi- 
tion being  the  important  considerations. 

1dy 
~=(a — y)k.    Integrating,  log   (a — y)  =  c — kx. 

y  =  total  yield  from  any  number  of  increments. 

x  =  amount  of  any  particular  fertilizer  constituent  utilized. 

a  =  maximum  yield  and  is  a  constant. 

k  :=  a  constant  depending  on  y  and  x,  variables. 

3Pfeiffer,  Th.,  Blanck,  E.,  and  Flugel,  M.,  W asset  und  Licht  als  Vege- 
tationsfaTctoren  und  ihre  Beziehungen  zum  Gesetze  vom  Minimum; 
Landw.  Ver.  Stat.,  Band  76.     Seite  211-223,  1912. 

Also,  Maze,  P.,  Recherches  sue  les  Relations  de  la  Plante  avec  les 
Elements  Nutritifs  der  Sol;  Compt.  Kend.,  Tome  154,  pp.  1711-1714, 
1912. 

•Potato  and  Corn  Fertilizer,  Golden  Harvest,  Ureka  Corn  Special, 
Blood  and  Bone,  Harvest  King,  Soil  Builder  and  the  like. 


THE  PRINCIPLES  OF  FERTILIZER  PRACTICE    479 

A  brand  of  fertilizer  is  usually  made  up  of  a  number  of 
materials  containing  the  important  nutrient  ingredients. 
These  materials,  already  described,  are  called  carriers.  The 
making-up  of  a  commercial  fertilizer  consists  in  mixing  the 
various  carriers  together  so  that  the  required  percentages  of 
ammonia,  potash,  and  phosphoric  acid  are  obtained,  care  being 
taken  that  no  detrimental  reaction  shall  occur  and  that  a 
physical  condition  consistent  with  easy  distribution  shall  be 
maintained.  Brands  of  fertilizer  put  out  by  reputable  com- 
panies carry  a  large  proportion  of  their  nutrients  in  a  readily 
available  form.  A  fertilizer  made  up  principally  of  dried 
blood,  tankage,  acid  phosphate,  and  kainit  or  muriate  of  pot- 
ash is  a  good  example  of  the  ordinary  composition  of  ready 
mixed  goods. 

The  various  brands  on  the  market,  besides  being  complete 
or  incomplete,  may  be  designated  as  high-grade  or  low-grade 
as  to  availability,  or  high-grade  or  low-grade  as  to  amount  of 
plant  nutrients  carried.  In  the  fertilizer  trade  the  terms 
generally  refer  to  the  latter  condition.  A  low-grade  fertilizer 
in  the  latter  sense  is  always  encumbered  with  a  large  amount 
of  inert  material,  called  filler,  which  adds  to  the  cost  of  mix- 
ing, transportation  and  handling.  A  low-grade  fertilizer  is 
generally  more  expensive  a  unit  of*  nutrient  obtained  than 
are  higher  grade  goods,  and  consequently  should  be  avoided. 

Fertilizer  concerns  have  always  found  it  more  profitable  to 
sell  ready  mixed  fertilizers  than  to  deal  in  the  separate  car- 
riers, such  as  dried  blood,  muriate  of  potash,  and  the  like.  Of 
late  years,  however,  it  has  been  possible  to  buy  the  separate 
materials.  The  conditions  during  the  World  War  greatly 
encouraged  the  application  directly  to  the  soil  of  separate 
carriers,  especially  acid  phosphate,  since  potash  was  almost 
unobtainable  and  nitrogen  fertilizers  were  very  high  in  price. 
The  use  of  phosphoric  acid  alone  is  often  much  more  eco- 
nomical and  rational  than  the  use  of  a  complete  mixture,  since 
the  nitrogen  removed  from  the  soil  by  normal  cropping  and 


480        NATURE  AND  PROPERTIES  OF  SOILS 

drainage  may  be  replaced  in  other  and  more  practical  ways. 
By  maintaining  the  soil  organic  matter  the  natural  supply  of 
potash  may  in  a  loamy  or  clayey  soil  often  be  so  influenced 
as  to  render  a  potash  fertilizer  unnecessary.  At  least  there 
may  be  enough  soil  potash  available  so  that  the  use  of  a  com- 
mercial form  will  not  be  profitable. 

270.  Fertilizer  inspection  and  control. — From  the  fact 
that  so  many  opportunities  are  open  for  fraud  either  as  to 
availability  or  as  to  the  actual  quantities  of  ingredients  pres- 
ent, laws  have  been  necessary  for  controlling  the  sale  of  fer- 
tilizers. These  laws  apply  not  only  to  the  ready  mixed  goods 
but  to  the  separate  carriers  as  well.  Most  states  have  such 
laws,  the  western  laws  generally  being  superior  to  those  in 
force  in  eastern  states,  where  the  fertilizer  sale  is  heavier. 
This  is  because  the  western  regulations  are  more  recent  and 
the  legislators  have  had  the  advantage  of  the  experience  gained 
where  fertilizers  have  long  been  used.  Such  laws  are  a  pro- 
tection not  only  to  the  public  but  to  the  honest  fertilizer  com- 
pany as  well,  since  spurious  goods  are  kept  off  the  market. 

Certain  provisions  are  more  or  less  common  to  most  fer- 
tilizer laws.  In  general,  all  fertilizers  selling  for  a  certain 
price  or  over  must  pay  a  state  license  fee  or  a  tonnage  tax  and 
print  the  following  data  on  the  bag  or  on  an  authorized  tag : 

1.  Number  of  net  pounds  of  fertilizer  to  a  package. 

2.  Name,  brand,  or  trade-mark. 

3.  Name  and  address  of  manufacturer. 

4.  Chemical  composition  or  guarantee. 

For  the  enforcement  of  such  laws  the  states  usually  pro- 
vide adequate  machinery.  The  inspection  and  analyses  may 
be  in  the  hands  of  the  state  department  of  agriculture,  of  the 
director  of  the  state  agricultural  experiment  station,  of  a 
state  chemist,  or  under  the  control  of  any  two  of  these.  In 
any  case,  a  corps  of  inspectors  is  provided,  the  members  of 
which  take  samples  of  the  fertilizers  on  the  market  throughout 
the  state.  These  samples  are  analyzed  in  laboratories  provided 


THE  PRINCIPLES  OF  FERTILIZER  PRACTICE    481 

for  the  purpose,  in  order  to  ascertain  whether  the  mixture  is 
up  to  guarantee.  The  expense  of  the  inspection  and  control  of 
fertilizers  is  usually  defrayed  by  the  license  fee  or  the  ton- 
nage tax. 

If  the  fertilizer  falls  below  the  guarantee, — allowing,  of 
course,  for  the  variation  permitted  by  law, — the  manufacturer 
is  subject  to  prosecution  in  the  state  courts.  A  more  effective 
check  on  fraudulent  guarantees,  however,  is  found  in  pub- 
licity. The  state  law  usually  provides  for  the  publication 
each  year  of  the  guaranteed  and  found  analyses  of  all  brands 
inspected.  Not  only  has  this  proved  effective  in  preventing 
fraud,  but  it  is  really  a  great  advantage  to  the  honest  manu- 
facturer, as  his  guarantees  receive  an  official  sanction.  The 
found  analysis  of  most  fertilizers  is  generally  above  the 
guarantee. 

271.  The  fertilizer  guarantee. — Every  fertilizing  mate- 
rial, whether  it  is  a  single  carrier  or  a  complete  ready -to-apply 
mixture,  must  carry  a  guarantee.  The  exact  form  is  gener- 
ally determined  by  the  state  in  which  the  fertilizer  is  offered 
for  sale.  The  content  of  nitrogen  is  almost  invariably  ex- 
pressed in  terms  of  ammonia  (NH3),  although  the  amount  of 
total  nitrogen  is  sometimes  required  in  addition.  The  phos- 
phorus is  quoted  in  terms  of  phosphoric  acid  (P205).  In 
some  cases,  a  bone-phosphate  of  lime  (B.  P.  L.  or  Ca3(P04)2) 
equivalent  is  included.  The  guarantee  of  a  simple  fertilizer 
material  is  easy  to  interpret,  since  the  name  of  the  material  is 
printed  on  the  bag  or  tag.  When  the  amount  of  the  nutrient 
element  carried  is  noted,  the  availability  and  general  value 
of  the  goods  is  immediately  known.  If  the  material  is  sodium 
nitrate  at  18  per  cent,  ammonia,  it  is  apparent  that  the  fer- 
tilizer is  high-grade  and  should  give  immediate  and  definite 
results  when  properly  applied  to  a  growing  crop. 

The  interpretation  of  a  complete  fertilizer  analysis  is  not 
as  easy,  however,  since  the  names  of  the  carriers  are  seldom 
included  in  the  guarantees.    The  simplest  form  of  guarantee 


482        NATURE  AND  PROPERTIES  OF  SOILS 

is  a  mere  statement  of  the  percentages  of  NH3,  P205  and  K20, 
as,  for  example,  a  2 — 8 — 2.1  This,  however,  is  too  brief  for  a 
guaranteed  analysis  on  goods  exposed  for  sale,  as  it  gives  no 
idea  whatsoever  regarding  the  solubility  of  the  materials.  As 
might  be  expected,  there  is  a  wide  range  in  the  character  of 
the  guarantees  required  by  the  various  states.  For  example, 
some  states  insist  on  the  statement  of  the  percentage  of  both 
nitrogen  and  ammonia,  while  others  insist  only  on  the  percent- 
age of  nitrogen.  Some  require  the  soluble,  the  reverted,  and 
the  total  phosphoric  acid,  while  others  require  only  the  soluble 
and  the  reverted.  As  to  potash,  in  some  cases  the  soluble 
must  be  stated,  while  in  other  cases  the  total  must  be  given.2 
In  general,  a  guarantee  should  show  not  only  the  amount 
of  the  various  constituents  but  also  their  form  or  availability. 
The  following  outline  analysis  is  excellent  in  this  respect : 

Percentage  of  NH3  as  nitrate.  Percentage  of  P205  soluble 

Percentage  of  NH3  as  ammonia.  in  water. 

Percentage  of  NH3  total.  Percentage  of  P206  reverted. 

Percentage  of  K20  water  soluble.  Percentage  of  P205  as 

Percentage  of  K20  as  chloride.  insoluble. 

272.  The  buying  of  mixed  goods. — The  successful  buying 
of  mixed  fertilizers  on  the  retail  market  depends  on  two 
things:  (1)  the  selection  of  a  composition  suitable  to  soil  and 
crop  with  carriers  of  known  value;  and  (2)  the  purchase  of 
high-grade  goods.  The  farmer  who  observes  these  points  will 
at  least  have  purchased  successfully.     Whether  he  obtains  a 

JIn  the  South,  the  order  is  different.  An  8-3-2  means  8  per  cent,  of 
P205,  3  per  cent,  of  NH3  and  2  per  cent,  of  K20. 

a  Below  is  the  guarantee  of  a  complete  fertilizer : 

Nitrogen    4.2% 

Equal  to  ammonia 5.0 

Soluble    P205 4.0 

Reverted   P205 2.0 

Available    P205 6.0 

Insoluble  P205 1.0 

Total   P205 7.0 

Water  soluble  K20 3.0 


THE  PRINCIPLES  OF  FERTILIZER  PRACTICE    483 

profit  from  the  use  of  the  fertilizer  depends  on  the  interrela- 
tion of  a  number  of  factors  more  or  less  variable  from  season 
to  season. 

The  selection  of  a  suitable  fertilizer,  as  to  carriers  and  com- 
position, entails,  after  the  need  of  the  crop  and  soil  are  de- 
cided, a  careful  study  of  the  guarantee.  Should  the  guarantee 
be  such  as  that  just  cited,  a  large  amount  of  information  is 
at  hand  concerning  the  forms  of  the  carriers  and  the  availa- 
bility of  the  important  constituents.  This  knowledge,  prop- 
erly correlated  with  the  probable  needs  of  the  crop  and  the 
soil,  will  determine  whether  a  particular  brand  should  be  pur- 
chased or  not.  The  real  question  here  is  not  so  much  the 
actual  quantities  of  the  elements  in  a  ton  of  the  fertilizer, 
as  it  is  their  balance  among  themselves.  The  actual  pounds  of 
nitrogen,  phosphoric  acid,  or  potash  applied  to  the  acre  can 
be  governed  by  the  rate  at  which  the  mixture  is  added. 

The  purchase  of  high-grade  goods  is  the  second  important 
point  to  be  considered.  Data  collected  from  practically  every 
State  show  that  the  higher  the  grade  of  the  fertilizer,  both  as 
to  availability  and  as  to  the  percentage  of  the  constituents 
carried,  the  greater  is  the  amount  of  nutrients  obtained  for 
every  dollar  expended.  Avoiding  the  abnormal  war 
prices,  the  following  data  from  Vermont 1  for  1909  seem 
representative : 

Table  XCVIII 


Cost  (in  Cents)  of  One  Pound  of 

Cents'  Worth 
of  Nutrients 

Mixed  Fertilizer 

NH3 

PA 

K20 

Keceived  for 

Every  Dollar 

Expended 

32 
26 
23 

7.6 
6.3 

5.7 

8.5 
7.0 
6.3 

50 

Medium  grade 

60 
67 

1  Hills,  J.  L.,  Jones,  C.  H.,  and  Miner,  H.  L.,  Commercial  Fertilizers; 
Vt.  Agr.  Exp.  Sta.,  Bui.  143,  pp.  147-149,  1909. 


484        NATURE  AND  PROPERTIES  OF  SOILS 

It  is  always  true  that  the  lower  the  grade  of  a  fertilizer  the 
higher  is  the  proportional  cost  of  placing  the  goods  on  the 
market.  In  other  words,  it  costs  just  as  much  a  ton  to  market 
a  low-grade  material  as  a  high-grade  one.  This  accounts  for 
the  fact  that  the  nutrients  are  cheaper  a  pound  in  a  high- 
grade  mixture,  and  that  the  value  received  for  every  dollar 
expended  is  greater. 

273.  The  purchase  of  unmixed  fertilizers. — There  has 
always  been  a  tendency  among  fertilizer  manufacturers  to 
discourage  the  purchase  by  the  farmer  of  the  separate  car- 
riers of  fertilizer  nutrients.  When  this  was  possible  the  fer- 
tilizer manufacturer  was  able  absolutely  to  control  the  mar- 
ket. By  selling  only  mixed  goods  the  manufacturer  could 
not  only  realize  a  profit  on  the  ingredients  themselves  but  a 
profit  on  the  mixing  in  addition.  In  order  to  escape  these 
costs  many  farmers  have  begun  the  practice  of  buying  the 
separate  carriers,  thus  avoiding  the  extra  charges.  In  manj^ 
cases,  the  mixing  on  the  farm  costs  nothing,  as  it  can  be  done 
in  winter  when  the  farm  work  is  not  pressing.  Home-mixing 
has  been  greatly  encouraged  by  post-war  conditions.  In  1920 
from  ten  to  twenty  dollars  a  ton  was  often  saved  on  a  high- 
grade  mixture  by  purchasing  the  carriers  separately. 

In  many  instances  the  fertilizing  materials  purchased  sepa- 
rately need  not  be  mixed  at  all,  thus  effecting  a  considerable 
saving  in  time  and  labor.  Acid  phosphate  is  generally  added 
separately,  especially  to  fall  wheat.  Bone-meal,  basic  slag, 
and  raw  rock  give  excellent  results  when  applied  with  farm 
manure.  Sodium  nitrate  and  ammonium  sulfate  give  good 
returns  as  a  top  dressing  on  meadows,  pastures,  and  small 
cereals,  especially  if  phosphates  have  been  added  at  some 
other  point  in  the  rotation.  When  farm  manure  is  available, 
the  use  of  acid  phosphate  with  lime  and  manure  in  a  legume 
rotation  is  generally  desirable.  Even  where  little  manure 
is  available,  the  application  of  sodium  nitrate  or  ammonium 
sulfate  as  a  top  dressing  for  meadows,  with  acid  phosphate  in 


THE  PRINCIPLES  OF  FERTILIZER  PRACTICE    485 

its  proper  place,  is  feasible.  The  purchase  of  expensive  ready- 
mixed  fertilizers  may  thus  be  avoided  without  necessitating 
home-mixing. 

For  vegetable  crops,  however,  especially  potatoes,  a  com- 
plete fertilizer  is  generally  advisable.  Home-mixing  is  in  such 
cases  necessary.  Special  soils  often  demand  a  complete  mix- 
ture. Muck  soils  generally  require  both  potash  and  phos- 
phoric acid,  while  sandy  soils,  especially  if  the  organic  matter 
is  low,  respond  to  a  mixture  carrying  all  three  of  the  fer- 
tilizer elements. 

As  might  be  expected,  this  practice  of  home-mixing  has  met 
with  much  opposition  from  manufacturers.  In  general,  it  is 
claimed  that  the  factory  goods  are  more  finely  ground  than 
those  mixed  by  the  farmer,  and  consequently  the  ready-mixed 
goods  are  not  only  more  uniform  but  also  in  better  physical 
condition.  Also,  the  manufacturer  is  able  to  treat  certain 
materials  with  acids,  and  thus  increase  their  availability. 
While  these  reasons  are  more  or  less  valid,  good  results  may 
be  expected  from  a  fertilizer  even  though  it  may  not  be  quite 
uniform,  as  the  soil  tends  to  equalize  this  deficiency.  More- 
over, by  screening  and  by  using  a  proper  filler,  a  farmer  can 
obtain  a  physical  condition  which  will  in  no  way  interfere 
with  the  drilling  of  the  material.  While,  obviously,  one  farm- 
er alone  cannot  afford  to  buy  small  lots  direct  from  the  whole- 
sale dealer  because  of  the  high  freight  charges,  this  objection 
is  being  met  by  organizations  of  various  kinds  whereby  the 
single  carriers  may  be  purchased  in  carload  lots  and  shipped 
directly  to  the  association. 

It  is  evident  that  by  purchasing  the  separate  carriers,  a 
farmer  is  able  to  obtain  pure  high-grade  material  at  a  reason- 
able price.  Even  if  the  fertilizers  are  not  home-mixed,  an 
educational  value  enters.  The  farmer  is  forced  to  study  the 
influence  of  the  materials  on  his  crops  more  closely  and  is  thus 
placed  in  a  position  to  make  changes  that  will  tend  to  a  higher 
efficiency  of  the  constituents.     The  chances  are  that  he  will 


486        NATURE  AND  PROPERTIES  OF  SOILS 

advantageously  alter  his  fertilizer  practice  as  the  rotation 
progresses  and  his  soil  changes  in  fertility. 

Such  arguments  do  not  always  mean,  however,  that  it  pays 
to  buy  the  separate  materials.  As  a  matter  of  fact,  in  many 
cases  it  does  not  pay,  especially  where  only  a  small  amount  of 
fertilizer  is  needed  and  it  is  impossible  to  cooperate  with 
other  farmers.  As  a  general  rule,  fertilizers  should  be  bought 
by  the  method  that  will  give  the  greatest  value  for  every  dollar 
expended,  providing,  of  course,  that  the  proper  material  is 
purchased.  Farmers  can  often  avail  themselves  of  the  advan- 
tage of  both  systems  by  asking  for  bids  from  various  manu- 
facturers on  carload  lots  of  mixed  goods  having  a  certain 
composition.  The  farmers  in  this  case  designate  the  carriers 
as  well  as  the  formula.  All  the  advantages  of  machinery  mix- 
ing may  thus  be  gained. 

274.  How  to  mix  fertilizers.1 — The  first  step  in  the  buy- 
ing of  the  separate  fertilizer  carriers  is  to  obtain  quotations 
which  should  state  the  price  a  ton,  the  composition,  and  the 
freight  rate.  With  this  information,  the  most  desirable  car- 
riers are  selected  and  the  amount  of  each  is  calculated.2    If 

1  Certain  materials  should  not  be  mixed,  especially  in  large  amounts. 
Thus  lime,  especially  the  oxide  and  hydroxide  forms  or  fertilizers  carry- 
ing lime  in  considerable  amount,  should  not  be  mixed  with  ammonium 
sulfate  and  animal  manures,  since  ammonia  is  likely  to  be  freed.  Such 
materials  should  be  kept  away  from  acid  phosphate  or  the  reversion  of 
the  latter  will  occur.  Calcium  carbonate  in  small  amounts,  however,  is 
often  mixed  with  fertilizers  carrying  acid  phosphate.  It  is  not  wise  to 
allow  moist  acid  phosphate  to  lie  in  contact  with  sodium  nitrate,  as  nitric 
acid  may  be  liberated  by  free  sulfuric  acid. 
a  Below  are  three  satisfactory  mixtures: 
2-12-0 

400  pounds  of  tankage. 
100  pounds  of  sodium  nitrate. 
1500  pounds  of  acid  phosphate  (16%P206). 
2-12-2 

320  pounds  of  tankage. 
100  pounds  of  ammonium  sulfate. 
1500  pounds  of  acid  phosphate  (16%P206). 
80  pounds  of  potassium  chloride. 
4-10-4 

150  pounds  of  sodium  nitrate. 
100  pounds  of  ammonium  sulfate. 


THE  PRINCIPLES  OF  FERTILIZER  PRACTICE    487 

the  materials  are  to  be  applied  separately,  the  rate  to  the  acre 
and  the  number  of  acres  must  be  known.  If  a  mixture  is  to  be 
made,  the  formula  of  this  mixture  must  be  decided  on  in  addi- 
tion. The  pounds  of  the  various  carriers  necessary  to  produce 
a  given  amount  of  a  certain  mixture  can  now  be  calculated. 
All  of  this  is  a  matter  of  good  judgement  and  careful  arith- 
metic.1 

With  the  separate  carriers  at  hand,  the  mixing,  if  necessary, 
is  quickly  accomplished.  All  that  is  needed  may  be  listed 
as  follows:  (1)  a  tight  floor,  (2)  a  coarse  sand  screen,  (3)  a 
tamper  or  grinder,  and  (4)  shovels,  a  rake,  and  like  tools. 
Since  the  pounds  of  fertilizer  are  quoted  on  each  bag,  weigh- 
ing is  unnecessary  in  making  up  a  given  amount  of  a  mixture 
having  a  certain  formula.  Bags  may  be  divided  into  half  or 
quartered  with  sufficient  accuracy. 

The  bulkiest  material  is  spread  on  the  floor  first  and  leveled 
uniformly  by  raking.  The  remaining  ingredients  are  then 
spread  in  thin  layers  above  the  first,  in  the  order  of  their  bulk. 
Beginning  at  one  side,  the  material  is  next  shoveled  over,  care 
being  taken  that  the  shovel  reaches  the  bottom  of  the  pile  each 
time.  The  pile  is  then  again  leveled,  and  the  process  is  re- 
peated a  sufficient  number  of  times  to  insure  thorough  mixing. 
Sometimes  a  mixing  machine  may  be  used  for  this  operation. 
For  storage  and  general  convenience,  the  fertilizer  may  be 
weighed  into  sacks  of  100  to  150  pounds  capacity  and  put  in  a 

240  pounds  of  tankage. 
100  pounds  of  dried  blood. 
1250  pounds  of  acid  phosphate  (16%P205). 
160  pounds  of  muriate  of  potash. 

*A  2-8-2  fertilizer  is  to  be  compounded  from  dried  blood  containing 
12%  NH3,  acid  phosphate  carrying  14%  P205  and  kainit  containing 
12%  K20.  In  one  ton  of  the  mixture  there  should  be  40  pounds  of  NH3, 
160  pounds  of  P205,  and  40  pounds  of  K20. 

40  -f-  ,12  =    333  lbs.  of  dried  blood. 
160-^.14=1142  lbs.  of  acid  phosphate. 
40-l-.12=    333  lbs.  of  kainit. 
192  lbs.  of  filler. 

2000  lbs.  total. 


488        NATURE  AND  PROPERTIES  OF  SOILS 

dry  place  until  needed.  Each  sack  should  be  labeled,  especi- 
ally if  different  mixtures  are  made. 

A  word  of  caution  should  be  inserted  here  regarding  the 
concentration  of  the  mixture.  Some  farmers,  in  order  to  les- 
sen the  work  of  mixing  and  application  in  the  field,  raise  the 
percentage  of  the  elements  exceedingly  high — a  condition  very 
likely  to  occur  when  high-grade  materials  are  used.  This 
sometimes  is  bad  practice,  in  that  it  may  interfere  with  ger- 
mination after  the  fertilizer  is  applied  and  may  also  injure 
the  young  plants.  Also,  it  is  likely  to  result  in  a  poor  physi- 
cal condition,  which  may  clog  the  drill,  and  in  uneven  distribu- 
tion, which  will  bring  about  a  lowered  efficiency  of  the  fertil- 
izer. The  use  of  sufficient  dry  finely  divided  filler  will  obviate 
such  dangers.1 

275.  The  choice  of  a  fertilizer.^-Two  primary  considera- 
tions must  be  observed  in  the  actual  utilization  of  fertilizers. 
The  first  of  these  has  to  do  with  the  composition  of  the  fer- 
tilizer and  its  suitability  to  soil  and  to  crop.  A  careful  study 
should  be  made  not  only  of  the  percentages  of  ammonia,  phos- 
phoric acid,  and  potash  but  also  the  availability  of  these  con- 
stituents. The  second  consideration  in  the  rational  use  of 
fertilizing  materials  is  in  regard  to  the  amounts  to  be  applied. 
As  much  care  and  good  judgment  are  necessary  in  handling 
a  single  carrier  as  a  complete  ready-mixed  material,  especially 
if  the  rotation  as  a  whole  is  considered. 

It  is  evident,  due  to  many  factors  that  cannot  be  controlled, 
that  fertilizer  formulae  for  different  crops  on  particular  soils 
are  difficult  to  determine.  In  fact,  such  data  can  never  be 
more  than  merely  suggestive.  Further,  the  best  quantity  of  a 
mixture  to  apply,  even  though  it  is  perfectly  balanced,  is  a 
figure  that  can  only  be  approximated.  Probably  the  largest 
percentage  of  the  fertilizer  waste  that  occurs  annually  can 

1Sand,  dry  soil,  saw  dust,  dry  muck,  and  even  ground  limestone,  if  in 
small  amounts,  may  be  used  as  fillers. 


THE  PRINCIPLES  OF  FERTILIZER  PRACTICE    489 

be  charged  to  this  factor.  Many  farmers  make  the  mistake  of 
applying  too  much  fertilizer.  Any  information  along  such 
lines,  however,  can  only  be  suggestive,  rather  than  literal, 
it  being  understood  that  the  general  formulae  suitable  to  vari- 
ous crops,  and  the  quantities  ordinarily  applied,  are  subject 
to  wide  variations. 

276.  Fertilizer  formulae.1 — In  the  popular  mind,  the  nu- 
trition of  a  plant  is  considered  as  similar  to  and  as  easy  as 
the  proper  feeding  of  an  animal.  With  animals,  the  food  is 
compounded  with  the  correct  balance  of  nutrients  and  if  other 
conditions  are  favorable,  normal  results  should  be  obtained. 
The  nutrition  of  a  plant  is  by  no  means  as  simple  as  the  proper 
feeding  of  an  animal.  In  the  first  place,  the  plant  receives 
most  of  its  nutrients  from  the  soil  and  air  and  not  from  the 
fertilizer,  since  the  latter  usually  merely  supplements  the  nu- 
trients already  present  in  the  soil.  Again,  the  food  for  the 
animal  remains  balanced  as  it  is  utilized.  In  the  case  of  plants, 
the  fertilizer  nutrients  undergo  great  changes  on  addition  to 
the  soil,  the  soil  influencing  the  availability  of  the  fertilizer 
as  well  as  the  fertilizer  influencing  the  soil  in  a  great  number 
of  different  ways.  Moreover,  the  question  of  fertilizer  resi- 
dues, especially  those  of  an  acid  nature,  is  always  paramount 
when  fertilizers  are  used  over  long  periods.  The  proper  for- 
mula for  a  given  crop  and  a  given  soil  under  a  probable  series 
of  weather  conditions  is  thus  more  or  less  of  a  guess  and  will 
always  remain  so. 

*The  following  example  of  fertilizers  similarly  named  but  carrying 
strikingly  different  guarantees  are  taken  from  Bull.  206  of  the  Vt. 
Agr.  Exp.  Sta. 

Potatoes  and  Maize  Potatoes  and  Tobacco 

4-7-8  2-  6-7 

4-8-4  2-  6-4 

4-8-0  2-12-0 

Vegetables  Top  Dressings 

3-  7-10  7-6-5 

4-  8-  4  7-6-2 
5-10-  0  7-6-0 


490        NATURE  AND  PROPERTIES  OF  SOILS 

In  spite  of  the  intangible  nature  of  the  question,  certain  gen- 
eral rules  seem  to  govern  the  compounding  and  use  of  fertiliz- 
ers. In  the  first  place,  the  ratio  of  the  nutrients  removed  by 
the  average  crop  bears  no  relation  to  the  composition  of  the 
fertilizer  usually  added.  This  is  to  be  expected  because  of 
the  complex  changes  that  the  fertilizer  undergoes  in  the  soil 
and  because  the  different  nutrients  influence  the  plant  di- 
versely. 

Table  XCIX 


Constituents 

Katio  of  the 

Constituents 

as  They  Occur 

in  the  Average 

Crop 

Ratio  of  the 

Constituents 

Carried  by  the 

Average 

Fertilizer 

Ammonia 

4 
2 
3 

0-2 

Phosphoric  acid 

16-8 

Potash 

0-2 

It  is  immediately  noticeable  that  the  ratios  of  the  ammonia 
and  potash  in  fertilizers  are  low.  The  ammonia  ratio  is  low 
because  of  the  ready  response  of  plants  to  nitrogen  and  the 
ease  with  which  this  constituent  is  lost  from  the  soil.  The 
potash  ratio  is  likewise  small  because  potassium  is  a  rather 
expensive  constituent  and  it  is  generally  better  if  possible  to 
render  available  by  suitable  means  that  which  is  already  in 
the  soil  than  to  buy  it  commercially.  The  phosphoric  acid 
is  high  in  comparison  with  the  ammonia  and  potash  because 
of  its  complex  reversion  in  the  soil  and  the  tendency  of  much 
of  it  to  remain  unavailable  for  long  periods  due  to  the  high 
absorptive  power  of  the  soil. 

The  following  data  may  now  be  presented.  These  for- 
mulae are  tentative  and  suggestive  only,  being  a  modification 
and  curtailment  of  certain  analyses  standardized  for  the  use 
of  fertilizer  manufacturers  in  the  United  States. 


THE  PRINCIPLES  OF  FERTILIZER  PRACTICE    491 


Table  C 

GROUP  I :      FODDER  AND  STAPLE  CROPS. 

Wheat  (fall)  Maize  Millet 

Oats  Barley  Beans  (field) 

Rye  (fall)  Buckwheat  Peas  (field) 


Soil 

Without 
Farm  Manure 

With 
Farm  Manure 

Sandy  soil 

2-10-6 

2-10-4 

{  2-12-2 

I  2-12-0 

0-12-4  1  or  Acid 
0-12-2  \  Phos. 

Acid  Phosphate 

Loamy  soil 

Clayey  soil 

Table  CI 

GROUP  II :      TOP  DRESSINGS. 


Soil 

Timothy, 
Orchard  Sod 

and 
Meadows  * 

Wheat,  Eye 

and  Oats 

for  Hay 

(Spring 

Dressing)* 

Pastures* 

AND 

Legumes 

Sandy  soil 

Loamy  soil 

Clayey  soil 

7-8-6 
7-8-3 
7-8-0 

7-8-3 
7-8-0 
7-8-0 

0-10-81  °^oid 

0-12-4     P£os: 
n  19  9  for  Basic 

°'12"2J     Slag 

*  Note. — Sodium  nitrate  or  ammonium  sulfate  may  be  used  alone  as 
a  top-dressing  on  all  of  these  crops  except  legumes. 


Table  CII 

GROUP  III:      VEGETABLES. 


1.  Extensively  —  Tomatoes, 
sweet  corn,  beets,  cab- 
bage, etc. 

Sandy   soil 3-10-6 

Loamy  soil 3-10-4 

rii                -i  (3-10-2 

Clayey   soil j  3-10-0 

All  root-crops  should  re- 
ceive at  least  2  per  cent, 
of  K20. 


2.  Intensively — Cabbage,  let- 
tuce,   celery,    asparagus, 


etc. 

Sandy 
Loamy 
Clayey 


soil 4-10-6 

soil 4-10-4 

soil 4-10-2 


The  ammonia  should  be  re- 
duced if  farm  manure  is 
used. 


492        NATURE  AND  PROPERTIES  OF  SOILS 


3.    Miscellaneous. 

fSandy    soil 7-6-5 

a.  Early  potatoes  * \  Loamy  soil 5-8-5 

[Clayey  soil 4-8-4 

fSandy   soil 5-8-7 

b.  Late  potatoes  * ]  Loamy  soil 4-8-6 

[Clayey  soil 4-8-4 

c.  General  trucking  *  on  sandy  soils  of  Atlantic 

seaboard 5-8-7 

*  Note. — Keduee  ammonia  if  farm  manure  is  used. 

In  this  table  of  suggested  formulae,  it  is  noticeable  that 
wherever  manure  is  used,  the  ammonia  is  reduced  or  even 
eliminated.  Ammonia  is  also  unnecessary  on  leguminous 
crops.  With  vegetables,  the  ammonia  is  usually  high.  Top 
dressings  for  pastures,  meadows,  and  cereals  in  the  spring 
should  always  carry  large  quantities  of  readily  available  nitro- 
gen. 

In  a  mixed  fertilizer,  the  phosphoric  acid  is  generally  high, 
for  reasons  already  explained.  Due  to  the  absorptive  power 
of  a  clay,  the  mixture  applied  to  such  a  soil  should  generally 
carry  more  phosphorus  than  that  added  to  a  sandy  soil.  Pot- 
ash is  usually  lower  in  a  fertilizer  for  clayey  soils,  due  to  the 
possibility  of  liberating  potassium  from  the  soil  itself  by  good 
soil  management. 

277.  Amounts  of  fertilizers  to  apply. — The  agricultural 
value  of  a  fertilizer  is  necessarily  a  variable  quantity,  since, 
in  applying  fertilizers,  a  material  subject  to  change  is  placed 
in  contact  with  two  wide  variables,  the  soil  and  the  crop. 
Moreover,  soil  conditions  are  constantly  changing,  thus  forc- 
ing a  modification  of  the  fertilizer  applied  to  the  same  soil 
bearing  the  same  crop  at  different  times.  The  factors  influ- 
encing the  efficiency  of  a  fertilizer  application  may  be  listed 
as  follows:  (1)  seed,  crop,  and  adaptation  of  crop,  (2)  weather 
conditions,  (3)  physical  condition  of  the  soil,  including  drain- 


THE  PRINCIPLES  OF  FERTILIZER  PRACTICE    493 

age,  (4)  organic  content  of  the  soil,  and  (5)  chemical  constitu- 
tion of  the  soil  and  its  reaction. 

Although  the  conditions  affecting  fertilizer  efficiency  have 
thus  been  so  briefly  disposed  of,  it  is  evident  that  they  are  of 
vital  importance  in  the  economical  utilization  of  fertilizing 
materials.  One  point  of  broader  scope  stands  out  particularly 
in  this  connection — the  necessity  of  putting  a  soil  in  any' 
given  climate  in  the  best  possible  condition  for  plant  growth. 
This  means  that  drainage,  lime,  organic  matter,  and  tillage, 
in  the  order  named,  must  be  raised  to  their  highest  perfection 
in  order  to  realize  the  best  results  from  fertilizers. 

Such  considerations  indicate  that  the  decision  as  to  the 
amount  of  a  single  carrier  or  of  a  mixed  fertilizer  that  should 
be  applied  will  be  difficult  and  probably  more  indefinite  than 
formula  selection.  In  fact,  the  amount  of  a  fertilizer  applied 
to  the  acre  is  more  vital  than  the  actual  chemical  composition, 
as  far  as  money  returns  are  concerned. 

With  all  the  groups  considered  above,  except  garden  and 
root-crops,  the  applications  are  generally  relatively  light,  rang- 
ing from  150  to  350  pounds  to  an  acre.  Where  excessive  vege- 
tative growth  is  required,  as  in  silage,  the  rate  may  be  in- 
creased to  500  pounds.  In  the  top  dressings  of  meadows  or 
grains,  the  rate  varies  from  100  to  200  pounds  an  acre.  Very 
often  this  dressing  is  sodium  nitrate  or  ammonium  sulfate 
alone.  With  garden  and  root-crops,  the  amount  of  fertilizer 
applied  is  very  large,  ranging  from  800  to  sometimes  as  high 
as  2000  pounds.  The  cropping  here  is  intensive,  and  the  ex- 
penditure for  fertilization  may  be  large  and  yet  yield  substan- 
tial profits. 

278.  The  law  of  diminishing  returns. — It  must  always 
be  remembered  that  in  fertilizer  practice  the  very  high  yields 
obtained  under  fertilizer  stimulation  are  not  always  the  ones 
that  give  the  best  returns  on  the  money  invested.  In  other 
words,  the  law  of  diminishing  returns  is  a  factor  in  the  in- 
fluence of  fertilization  on  crop  yield.     After  a  certain  point 


494        NATUEE  AND  PROPERTIES  OP  SOILS 

is  reached,  the  return  for  each  added  increment  of  fertilizer 
becomes  less  and  less.  It  is  evident,  therefore,  that  with  an 
excessive  application  of  any  mixture,  the  returns  to  an  in- 
crement will  at  last  become  so  small  that  the  increased  crop 
fails  entirely  to  pay  for  even  the  fertilizer,  not  to  mention 


<u 

0 

sc 

o 

d 

oo 

/bdO 

zooo 

Z40O 

POUflDS    OF    FLOATS     APPLIED  PER    s<7C&E 


44)0  800  /200  J600  2000  240t 

POUriDS    OF  FLOATS   APPLIED    PER  PCR.E 

Fig.  60. — In  the  upper  diagram  the  heavy  line  indicates  the  increase 
in  the  yield  of  maize  due  to  graduated  applications  of  floats.  The 
lower  diagram  shows  how  the  cost  of  the  fertilizer  approaches  and 
finally  exceeds  the  value  of  the  crop  as  the  applications  increase 
in  size. 


such  charges  as  cost  of  application,  harvesting  of  increased 
crop,  storage,  and  the  like.  The  application  of  moderate 
amounts  of  fertilizer  is  to  be  urged  for  all  soils  until  the  maxi- 
mum paying  quantity  that  may  be  applied  to  any  given  crop 
is  ascertained  by  careful  experimentation.  Over-fertilization 
probably  accounts  for  the  fact  that  such  a  large  proportion  of 


THE  PRINCIPLES  OF  FERTILIZER  PRACTICE    495 

the  fertilizer  sold  to  farmers  each  year  not  only  is  entirely 
wasted,  but  probably  in  some  eases  even  becomes  detrimental 
to  crop  yield. 

The  law  of  diminishing  returns  may  be  illustrated  by  data 
from  the  Cornell  University  Agricultural  Experiment x  Sta- 
tion. Floats  were  applied  at  different  rates  to  plats  receiv- 
ing a  uniform  dressing  of  farm  manure  at  the  rate  of  15 
tons  to  the  acre.  Table  CIII  shows  the  increased  yields  of 
maize  due  to  the  treatment  with  the  rock  phosphate.  Pre- 
war prices  were  used  in  the  calculations.     (See  Fig.  60.) 

Table  CIII 


Pounds  of  Floats 
to  the  Acre 

Maize 
(bus.) 

Maize 

(VALUE) 

Floats 
(cost) 

Difference 

200 

7.0 

8.3 

10.2 

12.7 

$4.62 
5.48 
6.73 

8.38 

$     .90 

1.80 

3.60 

10.80 

+$3.72 
+  3.68 
+  3.13 
—  2.42 

400 

800 

2400 

279.  Method  and  time  of  applying  fertilizers. — 
Although  considerable  emphasis  has  been  placed  on  the  selec- 
tion of  the  correct  fertilizer  formulae  and  on  the  adequate  and 
economical  amounts  to  use,  the  method  of  application  must 
not  be  lost  sight  of.  A  fertilizer  is  never  effective  unless  uni- 
formly distributed.  It  should  also  be  placed  in  the  soil  in 
such  a  position  that  it  will  stimulate  the  plant  to  the  best 
advantage. 

The  distribution  of  the  fertilizer  by  means  of  machinery 
is  much  more  satisfactory  than  is  broadcasting  by  hand, 
as  the  former  method  gives  a  more  uniform  distribution. 
Cereals  and  other  crops  are  now  usually  planted  with  a  drill 
or  a  planter  provided  with  an  attachment  for  dropping  the 
fertilizer  at  the  same  time  that  the  seed  is  sown,  the  fertilizer 

1Lyon,  T.  L.,  Soils  and  Fertilizers;  p.  216;  New  York,  1917. 


496        NATURE  AND  PROPERTIES  OF  SOILS 

being  by  this  method  placed  under  the  surface  of  the  soil. 
Broadcasting  machines  are  also  used,  which  leave  the  fer- 
tilizer uniformly  distributed  on  the  surface  of  the  ground, 
permitting  it  to  be  harrowed  in  sufficiently  before  the  seed  is 
planted,,  thus  preventing  injury  to  the  seed  by  the  chemical 
activity  of  the  fertilizing  material. 

Corn-planters  with  fertilizer  attachments  deposit  the  fer- 
tilizer beneath  the  seed,  thus  avoiding  a  possible  detrimental 
contact.  Grain-drills  do  not  do  this,  and,  where  the  amount 
of  fertilizer  used  exceeds  300  or  400  pounds  an  acre,  it  is 
better  to  apply  it  before  seeding.  Grass  and  other  small  seeds 
should  be  planted  only  after  the  fertilizer  has  been  mixed 
with  the  soil  for  several  days.  For  crops  to  which  large  quan- 
tities of  fertilizers  are  to  be  added,  especially  potatoes  and 
garden  crops,  it  is  desirable  to  drop  only  a  portion  of  the 
fertilizer  with  the  seed,  the  remainder  having  been  broad- 
casted by  machinery  and  harrowed  in  earlier. 

280.  Systems  of  fertilization. — During  the  evolution  of 
fertilizer  practice  since  the  middle  of  the  nineteenth  century, 
a  number  of  systems  of  applying  fertilizers  have  been  advo- 
cated and  in  many  cases  actually  followed.  Perhaps  the  first 
plan  to  be  suggested  was  the  single  element  system.  At  that 
time,  each  crop  was  supposed  to  respond  largely  to  one  par- 
ticular element.  Thus,  nitrogen  was  supposed  to  dominate 
wheat,  rye,  and  oats ;  phosphoric  acid,  to  dominate  maize, 
turnips,  and  sorghum;  and  potash  to  dominate  potatoes, 
clover,  and  beans.  Present  knowledge  of  plant  nutrition  and 
the  balancing  effects  of  fertilizer  nutrients  show  this  idea  to 
be  fallacious. 

The  supplying  of  abundant  minerals  as  a  fertilizer  system 
had  its  origin  from  the  fact  that  potash  and  phosphoric  acid 
are  relatively  cheap  and  are  rather  slowly  leached  from  the 
soil,  while  nitrogen  is  expensive  and  easily  lost  in  this  way. 
Such  a  plan,  therefore,  always  provides  plenty  of  potash  and 
phosphoric  acid,  which  are  to  be  balanced  each  season  with 


THE  PRINCIPLES  OF  FERTILIZER  PRACTICE    497 

sufficient  nitrogen  to  give  paying  yields.  While  this  system 
is  not  feasible  in  its  entirety  at  the  present  time,  the  prin- 
ciple involved  is  worthy  of  incorporation  with  more  economi- 
cal plans. 

A  system  based  on  the  amount  of  nutrients  removed  by 
crops  has  received  from  time  to  time  considerable  support. 
According  to  this  plan,  as  much  plant-food  material  is  added 
each  year  as  will  probably  be  taken  out  by  the  plant,  this 
being  determined  by  chemical  analyses  of  the  crop.  The 
system  not  only  overlooks  the  fact  that  diverse  plants  feed 
differently  on  the  same  soil,  but  that  the  same  crop  exhibits 
marked  variability  with  change  of  season  and  change  of  soil. 
Moreover,  no  allowance  is  made  for  losses  by  leaching,  which 
are  known  to  equal  at  times  the  losses  due  to  plant  absorption. 

In  trucking  or  in  general  farming  operations,  one  crop  is 
often  the  money  crop.  Naturally  its  stimulation  by  heavy 
fertilization  will  pay  better  than  applications  to  crops  that 
bring  less  on  the  market.  The  general  plan  in  this  system 
is  to  allow  the  crops  following  the  money  crop  to  utilize 
the  residuum.  When  this  residual  influence  works  out  fa- 
vorably, the  system  is  likely  to  be  a  profitable  one ;  but  when 
the  following  crops  fail  to  respond,  the  method  becomes 
wasteful  in  the  extreme. 

281.  Rational  fertilizer  practice. — In  the  selection  of  a 
system  that  will  result  in  an  effective  utilization  of  fertilizers, 
only  two  of  the  plans  described  above  need  be  considered.  In 
any  fertilizer,  phosphoric  acid  and  usually  potash  should 
always  be  present  in  amounts  sufficient  more  than  to  balance 
the  nitrogen,  since  the  activity  of  nitrogen  is  so  pronounced. 
Therefore,  a  scheme  that  calls  for  an  abundance  of  minerals 
is  a  sound  one.  This,  coupled  with  the  heavy  fertilization 
of  the  money  crop,  does  not,  however,  constitute  what  might 
be  considered  a  rational  system,  since  the  crops  that  follow 
may  or  may  not  be  adequately  supplied  with  nutrients. 

Not  only  must  the  soil,  the  crop  and  the  fertilizer  formula 


498        NATURE  AND  PROPERTIES  OF  SOILS 

and  amount  receive  careful  study,  but  the  rotation  should 
be  considered  in  addition.  This  is  a  fundamental  principle 
not  only  with  the  application  of  commercial  fertilizers  but 
with  liming  and  the  use  of  farm  manure  as  well.  The  care- 
ful fertilization  of  the  rotationT  with  special  reference  to  the 
money_crop,  is  the  only  rational  system  that  should  ordi- 
narily  be  employed,  since  it  not  only  cares  for  the  crop  on  the 
land  but  also  looks  to  those  that  are  to  follow.  The  atten- 
tiolTthat  must  necessarily  be  paid  to  the  fertility  of  the  soil 
in  such  a  system  insures  the  establishment  of  a  soil  manage- 
ment which  will  result  in  an  economical  use  of  the  plant 
nutrients,  while  at  the  same  time  the  yields  will  be  raised  and 
a  continuous  productivity  will  be  provided  for. 


CHAPTER  XXIV 
FARM  MANURE  1 

Of  all  the  by-products  of  the  farm,  barnyard  manure  is 
probably  the  most  important,  since  it  affords  a  means  where- 
by the  unused  portion  of  the  crop  may  become  a  part  of 
the  soil.  Its  use  not  only  makes  possible  a  return  to  the 
land  of  a  part  of  the  nutrients  previously  removed  by  the 
crop  but  also  permits  an  actual  gain  of  carbohydrate  ma- 
terials, the  elements  of  which  the  plant  obtains  not  from 
the  soil  but  from  air  and  water. 

This  country  has  already  entered  an  era  in  which  the  pre- 
vention of  agricultural  waste  is  becoming  necessary  and  a 
nearer  approach  to  a  self-sustaining  system  of  soil  manage- 
ment more  and  more  essential.  For  the  maintenance  of  fertil- 
ity, a  careful  handling  and  a  wise  utilization  of  all  the  manure 

1  The  following  publications  will  be  valuable : 

Ames,  J.  W.,  and  Gaither,  E.  W.,  Barnyard  Manure;  Ohio  Agr.  Exp. 
Sta.,  Bui.  246,  June  1912. 

Hart,  E.  B.,  Getting  the  Most  Profit  from  Farm  Manure;  Wis.  Agr. 
Exp.  Sta.,  Bui.  221,  June  1912. 

Thorne,  C.  E.,  Farm  Manures;  New  York,  1914. 

Beavers,  J.  C,  Farm  Manures;  Purdue  Univ.  Agr.  Exp.  Sta.,  Circ.  49, 
Mar.  1915. 

Burdick,  R.  T.,  Concerning  Farm  Manures;  Vt.  Agr.  Exp.  Sta.,  Bui. 
206,  June  1917. 

Fippin,  E.  O.,  Farm  Manure;  Cornell  Beading  Course  for  the  Farm, 
Lesson  127,  Aug.  1917. 

Weaver,  F.  P.,  Farm  Manure;  Pa.  State  Coll.,  Ext.  Circ.  No.  67,  Oct. 
1917. 

Brodie,  D.  A.,  Handling  Barnyard  Manure  in  Eastern  Pennsylvania; 
U.  S.  Dept.  Agr.,  Farmers'  Bui.  978,  July,  1918. 

Wiancko,  A.  T.,  and  Jones,  S.  C,  The  Value  of  Manure  on  Indiana 
Soils;  Purdue  Univ.  Agr.  Exp.  Sta.,  Bui.  222,  Sept.  1918. 

Duley,  F.  L.,  Handling  of  Farm  Manure;  Mo.  Agr.  Exp*  Sta.,  Bui. 
166,  Sept.  1919. 

499 


500        NATURE  AND  PROPERTIES  OF  SOILS 

produced  on  the  farm  are  vital.  Obviously  an  understanding 
is  necessary  regarding  the  character  and  composition  of  farm 
manure,  its  fermentative  and  putrefactive  changes,  its  losses 
in  handling  and  storage,  and  above  all  its  rational  use  as  an 
amendment  and  a  fertilizer.  This  need  appeals  not  only  to 
the  wide-awake  farmer  but  to  the  technical  man  as  well,  since 
in  the  use  of  farm  manures  theory  and  practice  widely  over- 
lap. 

282.  Composition  and  general  characteristics  of  farm 
manures. — The  term  farm  manure  may  be  employed  in  ref- 
erence to  the  refuse  from  all  animals  of  the  farm,  although, 
as  a  general  rule,  the  bulk  of  the  ordinary  manure  which 
ultimately  finds  its  way  back  to  the  land  is  produced  by 
cattle  and  horses.  This  arises  because  these  animals  consume 
the  greater  part  of  the  grain  and  roughage  on  the  average 
farm,  and  because  the  methods  of  handling  such  live-stock 
make  it  easier  and  more  practicable  to  conserve  their  excreta. 
Yard  manure  generally  refers  to  mixed  manures.  The  mixing 
usually  occurs  during  storage,  either  for  convenience  in  han- 
dling or  for  the  purpose  of  checking  losses  and  facilitating 
fermentation.  Thus,  horse  and  cow  manures  are  commonly 
mixed,  since  the  too  rapid  putrefaction  and  consequent  loss 
of  ammonia  in  the  former  is  checked,  while  at  the  same  time 
a  more  rapid  and  much  more  complete  decomposition  is  en- 
couraged in  the  latter. 

Ordinary  manure  consists  of  two  original  components, 
the  solid,  or  dung,  and  the  urine  in  about  the  rate  of  three 
to  one.  As  these  constituents  differ  greatly,  not  only  in  com- 
position but  also  in  physical  properties,  their  proportions 
must  appreciably  affect  the  quality  of  the  excreta  and  its  agri- 
cultural value.  Litter  added  for  bedding  or  for  absorptive 
purposes  is  almost  always  an  important  factor,  for  while  it 
prevents  losses  of  the  soluble  constituents,  it  may  at  the  same 
time  lower  the  value  of  the  product  for  a  unit  amount. 

While  compilations  of  available  data  on  the  composition  of 


FARM  MANURE 


501 


farm  manures  demand  liberal  interpretations,  they  afford 
considerable  light  regarding  the  differences  to  be  expected  be- 
tween excrement  from  various  animals. 

Table  CIV 

THE  COMPOSITION  OF  FRESH  MANURE.1 


Percentage  of 

HaO 

NH, 

P206 

K20 

[Solid,  80% 

Horse  ^Urine,  20% 

[Whole  manure .... 

fSolid,  70% 

Cow    ^Urine,  30% 

[Whole  manure .... 

fSolid,  67% 

Sheep  i  Urine,  33% 

[Whole  manure 

[Solid,  60%. 

Swine  \  Urine,  40% 

[Whole  manure 

75 
90 
78 

85 
92 
86 

60 

85 
68 

180 
97 

87 

.66 
1.63 

.84 

.48 
1.21 

.72 

.90 
1.63 
1.14 

.66 

.48 
.60 

.30 

Trace 

.25 

.20 

Trace 

.15 

.50 
.05 
.35 

.50 
.10 
.35 

.40 

1.25 

.55 

.10 

1.35 

.45 

.45 
2.10 
1.00 

.40 
.45 
.40 

Since  the  horse  does  not  ruminate  its  food,  the  manure  is 
likely  to  be  of  an  open  character.  It  is  also  fairly  dry,  as  is 
that  from  sheep,  the  urine  in  these  two  manures  making  up 
20  and  33  per  cent.,  respectively,  of  the  whole  product.  The 
complete  manure  from  these  two  animals  contains  78  and 
68  per  cent.,  respectively,  of  water — a  considerable  contrast 
to  the  cattle  and  swine  increments.  Cattle  and  swine  ma- 
nures, being  very  wet,  are  rather  solid  and  compact.  The  air, 
therefore,  is  likely  to  be  excluded  to  a  large  degree  and  de- 
composition is  relatively  slow.  They  are  usually  spoken  of 
as  cold  inert  manures  as  compared  with  the  dry,  open,  rapidly 
heating  excrements  obtained  from  the  horse  and  the  sheep. 

1Van  Slyke,  L.  L.,  Fertilisers  and  Crops,  p.  291;  New  York,  1912. 


502        NATURE  AND  PROPERTIES  OF  SOILS 

In  every  case  except  that  of  swine,  the  urine  is  much  the 
richer  than  the  dung  in  ammonia,  containing  on  an  average 
more  than  twice  as  much  when  compared  on  the  percentage 
basis.  The  urine  is  also  richer  in  potash  than  the  solid,  aver- 
aging for  the  four  classes  of  animals  1.29  per  cent,  as  com- 
pared to  0.34  per  cent,  contained  in  the  solid  manure.  Most 
of  the  phosphoric  acid,  however,  is  contained  in  the  solid  ex- 


TOTAL. 

TOTAL 

7VTPL 

AMMCN/A 

PS/05P/J0&C 

pOT/Q3/i 

06W> 

/QC/D 
025% 

0.5%. 

55% 


45% 


II 

& 


/oo% 


35% 


TXACE 


€5% 

- 


OL/A/G.   ue/rt£. 


PU/VG.     1/&A/E. 


PU/V6.     UE/NE. 


Fig.  61. — Diagram  showing  the  distribution  of  ammonia,  phosphoric 
acid  and  potash  between  the  dung  and  urine  of  average  farm 
manure. 

crement,  only  traces  being  found  in  the  urine  except  in  the 
case  of  swine.  It  is,  therefore,  evident  that  the  urine,  pound 
for  pound,  is  more  valuable  insofar  as  the  nutrient  elements 
are  concerned.  The  advantage  leans  heavily  toward  the 
urine  also  in  that  the  constituents  therein  contained  are  im- 
mediately available;  this  cannot  be  said  of  the  solid  manure. 
283.  Liquid  versus  solid  manure. — While  the  urine  car- 
ries more  nutrients  to  an  equal  weight  than  the  dung,  it  yet 
remains  to  be  seen  whether  in  the  total  excreta  voided  by  an 
animal  there  are  more  nutrients  in  the  urine  than  in  the  dung. 


FARM  MANURE 


503 


/ 


In  general,  more  solid  manure  is  excreted  than  liquid,  tend- 
ing to  throw  the  advantage  toward  the  former  as  a  carrier 
of  plant  nutrients.  The  following  table,  adopted  from  Van 
Slyke,1  bears  on  this  point: 

Table  CV 


DISTRIBUTION  OF  NUTRIENT  CONSTITUENTS  BETWEEN  THE  LIQUID 
AND   THE   SOLID    OF   WHOLE   MANURE. 


Animal 

Percentage 

of  Total 

NH3 

Percentage 

of  Total 

PA 

Percentage 

of  Total 

K20 

SOLID 

LIQUID 

SOLID 

LIQUID 

SOLID 

LIQUID 

Horse 

62 
49 
52 
67 

38 
51 
48 
33 

100 

100 

95 

88 

0 

0 

5 

12 

56 
15 
30 
57 

44 

Cow 

Sheep 

Swine 

85 
70 
43 

Average 

57 
55 

43 
45 

95 
100 

5 

0 

40 
35 

60 

Average  for  horse  and  cow 

65 

It  is  seen  here  that  a  little  more  than  one-half  the  am- 
monia, almost  all  the  phosphoric  acid,  and  about  two-fifths 
of  the  potash,  are  found  in  the  solid  manure.  Nevertheless, 
this  apparent  advantage  of  the  solid  manure  is  balanced  by 
the  ready  availability  of  the  constituents  carried  by  the  urine, 
giving  it  in  total  about  an  equal  commercial  and  agricultural 
value  with  the  solid  excrement.  Such  figures  are  suggestive 
of  the  care  that  should  be  taken  of  the  liquid  manure.  Its 
ready  loss  of  ammonia  by  fermentation  and  putrefaction,  and 
the  ease  with  which  all  its  valuable  constituents  may  escape 
by  leaching,  should  make  it  an  object  of  especial  regard  in 
handling.     (See  Fig.  61.) 

284.  Poultry  manure. — While  poultry  manure  is  often 
produced  on  the  farm  in  large  quantities,  it  is  not  included 
under  the  term  farm  manure,  which,  as  generally  used,  refers 

1Van  Slyke,  L.  L.,  Fertilizers  and  Crops,  p.  295;  New  York,  1912. 


504        NATURE  AND  PROPERTIES  OF  SOILS 


&■ 


to  the  excrement  of  the  larger  animals.    Its  general  composi- 
tion is  as  below,  the  data  being  averages  from  Thorne.1 

Table  CVI 

COMPOSITION   OF   POULTRY   MANURE. 


Condition 

Percentage  op 

H20 

NH, 

PA 

K30 

Whole  manure,  fresh 

Whole  manure,  air  dry 

57 

7 

1.31 

2.84 

.40 

.86 

.50 

1.08 

It  is  to  be  seen  that  poultry  manure  in  the  air-dry  state, 
the  condition  in  which  it  is  applied,  has  over  twice  the 
amounts  of  nutrients  carried  by  the  other  classes.  It  should 
be  applied  to  the  soil  at  at  least  one-half  the  rate  commonly 
recommended  for  ordinary  farm  manure.  Notwithstanding 
its  ease  in  handling  and  its  great  value,  poultry  manure  re- 
ceives less  care  and  attention  than  any  other  produced  on  the 
farm. 

285.  Farm  manure — a  direct  and  indirect  fertilizer. — 
Farm  manure,  when  applied  to  the  land,  ordinarily  fulfills 
two  functions  which  are  usually  not  so  distinctly  developed  in 
one  material — that  of  a  direct  and  indirect  fertilizer.  Mixed 
farm  manure  ready  to  apply  to  the  land  contains  on  the  aver- 
age .6  per  cent,  of  ammonia,  .25  per  cent,  of  phosphoric  acid 
and  .5  per  cent,  potash.2    It  is  obviously  a  low-grade  fertilizer 

thorne,  C.  E.,  Farm  Manures,  p.  90;  New  York,  1914.    Also, 

Storer,  F.  H.,  Agriculture,  Vol.  I,  p.  613;  New  York,  1910. 

Vorhees,  E.  B.,  Ground  Bone  and  Miscellaneous  Samples;  N.  J.  Agr. 
Exp.  Sta.,  Bui.  84,  1891. 

Goessman,  C.  A.,  Mass.  Agr.  Exp.  Sta.,  Bui.  37,  1890,  and  Bui.  63, 
1896. 

2  See  Analyses,  Storer,  F.  H.,  Agriculture,  pp.  237-248;  New  York, 
-iy  J.U. 

Thorne,  C.  E.,  Farm  Manures,  pp.  89-93;  New  York,  1914. 

Aikman,  C.  M.,  Manure  and  Manuring,  pp.  279-292;  Edinburgh  and 
London,  1910.  *^  6 

J*ob€rts,  I.  P.,  The  Fertility  of  the  Land,  pp.  159-182;   New  York, 


FARM  MANURE  505 

both  as  to  the  amounts  of  nutrients  carried  and  as  to  their 
availability.  Because  of  the  large  acre  applications  of  ma- 
nure commonly  made,  the  fertilizer  constituents  added  in  ma- 
nure are  considerable.  Ten  tons  of  farm  manure,  even  if  only 
one-half  its  ammonia,  one-sixth  of  its  phosphoric  acid  and  one- 
half  of  its  potash  were  readily  available,  are  equal  in  fertil- 
izing value  to  333  pounds  of  sodium  nitrate,  52  pounds  of 
acid  phosphate,  and  416  pounds  of  kainit.  This  equiva- 
lent to  the  addition  of  801  pounds  of  a  readily  available  mix- 
ture of  fertilizer  salts.  This  calculation,  however,  ignores 
an  equal  quantity  of  nutrients  which  remain  in  the  soil  as 
a  residuum  and  may  be  used  by  succeeding  crops.  This  resi- 
dual effect  of  manure  is  generally  a  paying  one  during  the 
period  of  an  ordinary  rotation. 

Farm  manure  acts  as  an  indirect  fertilizer  in  that  it  adds 
to  the  soil  organic  matter  and  thus  improves  the  physical 
condition  of  the  land.  While  it  may  not  increase  the  organic 
matter  of  the  soil,  because  of  the  loss  of  carbon  by  exhalation 
and  leaching  during  the  period  of  crop  growth,  its  use  materi- 
ally influences  the  rate  of  reduction.  Better  aeration,  drain- 
age and  bacterial  activity  *  of  necessity  result  from  such  an 
addition.  The  influence  of  manure  on  the  availability  of 
the  mineral  constituents  of  the  soil  is  not  the  least  of  its 
indirect  actions.  The  fact  that  rock  phosphate  when  mixed 
with  manure  seems  to  have  a  higher  availability  bespeaks 
a  considerable  solvent  activity.  The  tendency  of  farm 
manure  to  alleviate  toxic  conditions,  such  as  alkali  and  acid- 
ity, deserves  attention. 

286.  Outstanding  characteristics  of  farm  manure. — As 
farm  manure  is  essentially  a  fertilizer,  whether  it  is  pro- 
duced on  the  farm  or  purchased  outright,  it  is  logical  to  con- 
trast it  with  the  ready-mixed  materials  on  the  market.     In 

1  Conn,  H.  J.,  and  Bright,  J.  W.,  Ammonification  of  Manure  in 
Soil;  Jour.  Agr.  Ees.,  Vol.  XVI,  No.  12,  pp.  313-350,  March,  1919. 

Fulmer,  H.  L.,  and  Fred,  E.  B.,  Nitrogen  Assimulating  Organisms  in 
Manure;  Jour  Bact.,  Vol.  II,  No.  4,  pp.  423-434,  1917. 


506        NATURE  AND  PROPERTIES  OF  SOILS 

such  a  comparison,  five  characteristics  are  outstanding:  (1) 
the  moist  condition  of  manure,  (2)  its  low  grade,  (3)  its 
unbalanced  nutrient  condition,  (4)  its  variability,  and  (5) 
its  rapid  fermentative  and  putrefactive  processes.  These 
characteristics,  neither  present  nor  desirable  in  ordinary  fer- 
tilizers, place  farm  manure  in  a  class  by  itself  as  to  its  hand- 
ling, storage,  and  field  utilization. 

Of  the  above  points,  the  first  three  may  be  disposed  of 
quickly.  Average  farm  manure,  whether  fresh  or  well-rotted, 
contains  from  70  to  85  per  cent,  water.  A  ton  of  average 
mixed  manure  when  applied  to  the  land  carries  but  12  pounds 
of  ammonia,  5  pounds  of  phosphoric  acid,  and  10  pounds  of 
potash  to  the  ton.  Approximately  one-half,  one-sixth,  and 
one-half,  respectively,  of  these  constituents  are  readily  avail- 
able. Farm  manure  is,  therefore,  low-grade  on  two  distinct 
counts.  Moreover,  its  readily  available  nutrients  approximate 
a  ratio  of  about  6-1-6,  a  marked  contrast  to  the  2-8-2  often 
given  for  the  average  ready-mixed  fertilizers  on  the  market. 
Obviously,  manure  is  much  too  low  in  phosphoric  acid  for  its 
content  of  active  ammonia  and  potash.  The  variability  and 
decomposition  of  farm  manure  will  be  considered  separately. 

287.  Variability  of  farm  manure. — The  manure  pro- 
duced on  the  average  farm  will  obviously  vary  in  its  char- 
acter and  composition  from  time  to  time.  The  factors  re- 
sponsible may  be  listed  as  follows:  (1)  class  of  animal,  (2) 
age,  condition,  and  individuality  of  animal,  (3)  food,  and 
(4)  the  handling  and  storage  which  the  manure  receives  be- 
for  it  is  placed  on  the  soil. 

The  differences  in  composition  due  to  class  of  animal  have 
been  adequately  disposed  of  in  previous  paragraphs.  In  ad- 
dition, it  is  obvious  that  the  age  and  condition  of  any 
animal  within  a  class  will  influence  the  character  of  the  ex- 
crement produced.  A  young  animal  gaining  in  bone  and 
muscle  will  retain  large  amounts  of  nutrients,  and  the  manure 
will  be  correspondingly  poorer  in  dry  matter,  nitrogen,  lime, 


FARM  MANURE 


507 


phosphoric  acid,  and  potash.  A  fattened  animal  on  a  main- 
tenance ration  will  return  almost  all  of  the  nutrient  value  of 
the  original  food. 

Since  the  animal  will  retain  only  a  certain  quantity  of 
the  important  food  elements,  it  is  only  reasonable  to  assume 
that  the  richer  the  food,  the  richer  will  be  the  corresponding 
excrement.  The  following  data  from  Ohio 1  obtained  with 
western  lambs  substantiate  this  assumption: 

Table  CVII 

EFFECT  OF  RATION   ON   MANURIAL   COMPOSITION. 


Eation 


Percentage  of 


NH3 


PA 


K20 


Corn  and  mix  hay 

Corn,  oil  meal  and  hay .  .  . 
Corn,  oil  meal  and  clover 


1.80 

1.87 
2.03 


.51 
.53 

.58 


1.33 
1.22 
1.25 


While  the  factors  just  disposed  of  cause  some  variation  in 
farm  manure,  the  character  of  the  product  as  it  goes  on  to 
the  land  is  determined  in  large  degree  by  the  handling.  Tight 
floors  and  proper  bedding  hold  the  liquid  manure  in  contact 
with  the  solid  and  thus  maintain  the  proportion  of  valuable 
constituents.  A  neglect  of  these  two  conditions  means  a  grave 
loss  in  value.  The  storage  of  manure,  when  it  is  not  taken 
directly  to  the  field,  always  results  in  loss  not  only  of  organic 
matter,  but  of  ammonia  and  minerals  as  well.  As  more  than 
one-half  of  the  ammonia  and  potash  are  water-soluble,  seri- 
ous loss  is  unavoidable.  Such  losses  over-ride  other  causes  of 
variation.  The  influence  of  storage  is  clearly  shown  by  the 
following   figures   from    Schutt 2   on   mixed   horse    and    cow 

1  Thome,  C.  E.,  and  others.  The  Maintenance  of  Fertility;  Ohio  Agr. 
Exp.  Sta.,  Bui.  183,  1907. 

2 Schutt,  M.  A.,  Barnyard  Manure;  Canadian  Dept.  Agr.,  Centr.  Exp. 
Farm,  Bui.  31,  1898. 


6 


508        NATURE  AND  PROPERTIES  OF  SOILS 

manure.  The  protected  manure  was  stored  in  a  bin  under 
a  shed.  The  exposed  sample  was  in  a  similar  bin  but  unpro- 
tected. 

Table  CVIII 

LOSS    OP    CONSTITUENTS     FROM     PROTECTED     AND    UNPROTECTED 

MANURE. 


Constituents 

Percentage  Loss  at 
End  of  Six  Months 

Percentage  Loss  at 
End  of  One  Year 

PROTECTED 

EXPOSED 

protected 

EXPOSED 

Loss  of  organic  matter 
Loss  of  NH3 

58 

19 

0 

3 

65 
30 
12 

29 

60 

23 

4 

3 

69 
40 

Loss  of  P205 

Loss  of  K20 

16 
36 

288.    The  fermentation  and  putrefaction  of  manure.1 — 

In  the  process  of  digestion,  the  food  of  animals  becomes  more 
or  less  decomposed.  This  condition  comes  about  partly  be- 
cause of  the  digestive  process  and  partly  from  the  bacterial 
action  that  takes  place.  Of  these  two  influences  within  the 
animal,  bacterial  activities  are  probably  of  the  greater  im- 
portance as  far  as  the  breaking-up  of  the  complicated  food- 
stuffs is  concerned.  The  fresh  excrement,  then,  as  it  comes 
from  the  stable,  consists  of  decayed  or  partially  decayed 
plant  materials,  with  a  certain  amount  of  broken-down  animal 
tissue  and  mucus.  This  is  more  or  less  intimately  mixed  with 
litter  and  the  whole  mass  is  moistened  with  the  liquid  excre- 
ment carrying  considerable  quantities  of  soluble  nitrogen  and 
potash.    This  mass  of  material,  ranging  from  the  most  com- 

1Good  general  discussions  may  be  found  as  follows:  Lipman,  J.  G., 
Bacteria  in  Belation  to  Country  Life,  pp.   303-356;    New  York,   1911. 

Hall,  A.  D.,  Manures  and  Fertilizers,  pp.  184-210;   New  York,  1921. 

For  a  technical  discussion  see  Eussell,  E.  J.,  and  Richards,  E.  H.,  The 
Changes  Taking  Place  During  the  Storage  of  Farm  Manure;  Jour.  Agr. 
Sci.,  Vol.  VIII,  Part  4,  pp.  495-563,  Dec,  1917. 


FARM  MANURE  509 

plex  compounds  to  the  most  simple,  is  teeming  with  bacteria,1 
especially  those  that  function  in  fermentation  and  putrefac- 
tion. The  number  very  often  runs  into  billions  to  a  gram 
of  excrement.  In  such  an  environment,  it  is  little  wonder 
that  biological  changes  go  on  rapidly.  These  changes  may  be 
grouped  for  convenience  of  discussion  under  two  heads — 
aerobic  and  anaerobic. 

When  manure  is  first  produced,  it  is  likely  to  be  rather 
loose,  and  if  allowed  to  dry  at  once  it  becomes  well  aerated. 
The  first  bacterial  action  is,  therefore,  likely  to  be  rather 
largely  aerobic  in  nature.  Transformations  are  very  rapid 
and  are  accompanied  by  considerable  heat,  ranging  from  100° 
to  150°  F.  and  sometimes  higher.  This  action  falls  largely 
on  the  simple  nitrogenous  compounds,  although  the  more 
complicated  nitrogenous  and  non-nitrogenous  constituents  are 
by  no  means  unaffected.  Urea  is  particularly  influenced  by 
aerobic  activities  and  quickly  disappears  from  well-aerated 
manure. 

CON2H4  +  2H20  =  NHJ2C03 
NH4)2C03  =  NH3  +  C02  +  H20 

Thus  nitrogen  may  be  rapidly  lost  from  manure  by  allow- 
ing excessive  aerobic  decay  and  decomposition  to  proceed. 
This  loss,  however,  is  often  somewhat  checked  by  the  oxidiz- 
ing influence  of  nitrifying  bacteria,  especially  in  the  outer 
portions  of  the  manure  pile.  The  evolution  of  carbon  dioxide 
which  goes  on  continuously  indicates  how  extensively  the 
organic  matter  of  the  manure  is  suffering  through  biological 
activity. 

As  the  manure  becomes  compacted,  especially  if  it  is  left 

moist,  oxygen  is  gradually  excluded  from  the  heap  and  its 

place  is  taken  by  carbon  dioxide,  which  is  given  off  during 

the  progress  of  any  form  of  bacterial  activity.     The  decay 

now  changes  from  aerobic  to  anaerobic,  it  becomes  slower,  and 

1  Murray,  T.  J.,  Study  of  the  Bacteria  of  Fresh  and  Decomposing 
Manure;  Va.  Agr.  Exp.  Sta.,  Bui.  15,  Part  II,  1917. 


510        NATURE  AND  PROPERTIES  OF  SOILS 

the  temperature  falls  to  as  low  as  80°  or  90°  F.  New  organ- 
isms may  now  function,  although  many  of  those  active  under 
aerobic  conditions  may  continue  to  be  effective.  The  prod- 
ucts become  changed  to  a  considerable  degree.  Carbon  diox- 
ide, of  course,  continues  to  be  evolved  in  large  amounts,  but 
instead  of  ammonia  being  formed,  the  nitrogenous  matter  is 
converted  into  the  usual  putrefactive  products,  such  as  indol, 
skatol,  and  the  like.  If  sufficient  reduction  occurs,  free  nitro- 
gen may  escape. 

The  carbonaceous  matter  is  resolved  into  numerous  hydro- 
carbons, of  which  methane  (CH4)  is  prominent;  and  as  a  by- 
product of  the  breaking-down  of  the  proteins,  hydrogen  sul- 
fide (H2S)  and  sulfur  dioxide  (S02)  are  evolved.  The  com- 
plex nitrogenous  and  carbohydrate  bodies  are  attacked  with 
the  splitting-off,  not  only  of  simpler  materials,  but  often  of 
those  more  complex.  Such  compounds  may  be  listed  in  gen- 
eral as  organic  acids  and  humous  bodies.  They,  of  course,  ul- 
timately succumb  to  simplification. 

The  general  changes1  in  any  manure  pile  can  readily  be 
recapitulated.  First  is  the  aerobic  action,  with  the  escape  of 
ammonia  and  carbon  dioxide.  Next  the  manure  is  wetted, 
it  compacts,  and  the  slow,  deep-seated  decay  sets  in  with  a 
simplification  of  some  compounds,  with  the  production  of 
acids,  and  with  a  gradual  formation  of  humous  materials. 
As  the  manure  becomes  alternately  wet  and  dry,  the  two  gen- 
eral processes  may  follow  each  other  in  rapid  succession,  the 
anaerobic  bacteria  attacking  the  complex  materials,  the 
aerobic  affecting  both   the   complex  and  the   simpler   com- 

1The  proteid  compounds,  which  are  the  most  important  group  in  farm 
manures,  split  up  in  the  soil  or  compost  heap  into  amino-acids.  These 
amino-acids  undergo  deaminisation  and  decarboxylation.  The  former 
takes  place  either  under  aerobic  or  anaerobic  conditions  producing  am- 
monia and  a  complex  acid.  The  decarboxylation  occurs  only  when  oxygen 
is  excluded  giving  either  ammonia  and  an  organic  acid  as  in  deaminisa- 
tion, or  carbon  dioxide  and  a  complex  amine,  which  may  be  rather  stable. 
Deaminisation  and  decarboxylation  go  on  together,  the  former  generally 
predominating. 


FARM  MANURE  511 

pounds.  Carbon  dioxide  is  given  off  continuously  during  the 
process.  Some  gaseous  nitrogen  as  well  as  ammonia  is  prob- 
ably lost  because  of  the  rapid  alternations  of  conditions.1 

289.  Effect  of  decomposition  on  the  value  of  manure. — 
Because  of  the  great  loss  of  carbon  dioxide  and  water  dur- 
ing the  decay  processes,  there  is  considerable  change  in  bulk 
of  the  manure.  Fresh  excrement  loses  from  20  to  40  per  cent, 
in  bulk  by  partial  rotting  and  50  per  cent,  by  becoming  more 
thoroughly  decomposed.  This  means  that  1000  pounds  of 
fresh  manure  may  be  reduced  to  800,  600,  or  500  pounds, 
according  to  the  degree  of  change  it  has  undergone. 

It  is  often  argued  that  if  the  manure  is  properly  stored, 
this  rapid  loss  of  carbon  dioxide  and  water  will  raise  the 
percentage  amounts  of  the  fertilizer  elements.  The  simplify- 
ing action  of  the  anaerobic  fermentation  and  putrefaction 
is  an  additional  reason  for  expecting  better  results  from  well- 
rotted  manure  when  it  is  compared,  ton  for  ton,  with  the 
fresh  material.  In  practice,  however,  the  losses  in  handling 
due  to  leaching  and  fermentation  are  so  dominant  as  to  place 
well-rotted  manure  at  a  disadvantage  except  on  sandy  land  or 
for  garden  and  trucking  purposes.  At  the  Ohio  Experiment 
Station,2  yard  and  stall  manure  were  compared  in  equal 
amounts  in  a  three-year  rotation  of  maize,  oats,  and  hay.  The 
yard  manure  was  exposed  for  some  months  in  the  open,  while 
the  stall  manure  came  directly  from  the  stable.  The  increase 
due  to  yard  manure  is  taken  as  100  in  each  case.  (Table  CIX, 
p.  512.) 

A  change  of  a  biological  nature  which  sometimes  takes 
place  in  loose  and  rather  dry  manure  is  fire-fanging.  Many 
farmers  consider  this  to  be  due  to  actual  combustion,  as  the 

1  Under  the  alternating  aerobic  and  anaerobic  conditions  found  in  the 
average  manure  pile,  gaseous  nitrogen  seems  to  be  lost  in  considerable 
amounts.  This  loss  probably  occurs  through  the  oxidation  of  ammonia 
to  nitrites  or  nitrates  with  a  later  reduction  of  the  nitrogen  so  carried 
to  a  free  state. 

2Thorne,  C.  E.,  The  Maintenance  of  Fertility;  Ohio  Agr.  Exp.  Sta., 
Bui.  183,  p.  209,  1907. 


512        NATURE  AND  PROPERTIES  OF  SOILS 


Table  CIX 

COMPARATIVE  YIELDS  FROM  YARD   AND  STALL  MANURE. 


Average  Increase  to  the  Acre 

MANURE 

Corn,  10  Years 

Wheat,  10  Years 

Hay, 

GRAIN 

STOVER 

grain 

STOVER 

6  Years 

Stall 

100 

72 

100 
68 

100 
85 

100 

87 

100 

Yard 

54 

manure  is  very  light  in  weight  and  has  every  appearance  of 
being  burned.  This  condition,  however,  is  produced  by  fungi 
instead  of  bacteria,  and  the  dry  and  dusty  appearance  of  the 
manure  is  due  to  the  mycelium,  which  penetrates  in  all  di- 
rections and  uses  up  the  valuable  constituents.  Manure  thus 
affected  is  of  little  value  either  as  a  fertilizer  or  as  a  soil 
amendment. 

290.  Evaluation  of  farm  manure. — For  purposes  of  com- 
parison, experimentation,  and  sale,  farm  manures  are  often 
evaluated  in  a  way  similar  to  that  used  with  commercial  fer- 
tilizers. The  great  difficulty  here  lies  in  arriving  at  prices 
for  the  important  constituents  which  are  at  all  comparable 
with  the  value  of  the  manure  in  the  field.  If  the  value  of  the 
ammonia  in  manure  is  arbitrarily  placed  at  15  cents  a  pound, 
phosphoric  acid  at  5  cents,  and  potash  at  8  cents,  certain 
tentative  calculations  may  be  made.  While  such  assumptions 
do  not  establish  the  commercial  value  either  of  fresh  or 
stored  manure,  they  are  of  some  use  in  comparisons  and  gen- 
eralizations. The  average  manure,  as  it  goes  on  the  land,  car- 
ries about  12  pounds  of  ammonia,  5  pounds  of  phosphoric 
acid,  and  10  pounds  of  potash.  Using  the  prices  above,  such 
manure  is  worth  commercially  about  $3.00  a  ton. 

The  commercial  evaluation  must  be  applied  with  care  be- 
cause of  the  many  factors  tending  to  vary  the  composition  of 


FARM  MANURE  513 

the  excrement.  Litter,  particularly,  will  exert  a  great  influ- 
ence in  this  direction.  Moreover,  this  mode  of  evaluation 
must  never  be  confused  with  the  much  more  important  figure 
known  as  the  agricultural  value  of  a  manure.  The  former 
is  based  on  composition  and  assumed  values  of  doubtful  char- 
acter. The  latter  arises  from  the  effect  of  the  manure  on  crop 
yield.  Obviously,  a  rational  utilization  of  farm  manure,  as 
with  any  fertilizer,  should  strive  for  the  highest  return  to 
an  increment  applied.  A  very  good  comparison  between 
commercial  and  agricultural  values  may  be  cited  from  the 
Ohio  experiments  1  with  manure.  The  manure  was  treated  in 
various  ways  and  applied  to  maize  in  a  three-year  rotation 
of  maize,  wheat,  and  hay.  Twenty-six  crops  were  grown. 
The  commercial  evaluation  is  taken  as  100  in  every  case. 

Table  CX 

COMMERCIAL  AND  AGRICULTURAL  EVALUATION  OP  FARM   MANURE. 


Manure 

Commercial 
Value 

Agricultural 
Value 

Yard  manure,  untreated 

100 
100 
100 
100 
100 

152 

Yard  manure,  plus  floats 

162 

Yard  manure,  plus  acid  phosphate.  . 

Yard  manure,  plus  kainit 

Yard  manure,  plus  gypsum 

222 
192 
186 

291.  Amount  of  manure  produced  by  farm  animals. — A 
well-fed  moderately  worked  horse  will  produce  daily  from 
45  to  55  pounds  of  manure,  of  which  10  to  12  pounds  is 
urine.  A  dairy  cow,  having  a  greater  food  capacity,  will  ex- 
crete from  70  to  90  pounds  during  the  same  period,  of  which 
20  to  30  pounds  is  liquid.  Farm  animals,  especially  sheep 
and  swine,   vary   so  much  in  size  that   a  thousand  pound 

1  Thome,  C.  E.,  and  others,  The  Maintenance  of  Fertility;  Ohio  Agr. 
Exp.  Sta.,  Bui.  183,  pp.  206-209,  1907. 


514        NATURE  AND  PROPERTIES  OF  SOILS 

weight  of  animal  is  the  only  fair  and  logical  basis  of  calcu- 
lation. 

Table  CXI 

MANURE    EXCRETED    BY    VARIOUS   FARM    ANIMALS    TO    THE    1000 
POUNDS  LIVE  WEIGHT. 


Animal 

Pounds 
a  Day 

Tons  a 
Year 

Horse  1  .        

50 
70 
40 
85 
34 
23 

9.1 

Cow 2    . 

12.7 

Steer 3   

7.3 

Swine  4   

15.5 

Sheep  5  

6.2 

Poultry  5   

4.2 

It  is  to  be  noted  that  these  figures  do  not  include  litter, 
which,  in  cases  of  horses  and  cattle,  will  range  from  15  to  20 
per  cent,  of  the  weight  of  the  pure  excrement.  A  working 
horse  would  be  expected  to  produce  from  10  to  11  tons  of 
average  manure  a  year,  while  a  dairy  cow  on  the  same  basis 
would  produce  14  or  15  tons. 

Rough  calculations  as  to  manurial  production  from  horses 
and  cattle  may  be  made  from  the  food  consumed  by  these 
animals.6  It  is  assumed  that  50  per  cent,  of  the  dry  matter  of 
the  food  appears  in  the  excrement  and  that  the  necessary 
bedding  equals  one-half  of  the  dry  matter  of  the  excrement. 

1  Roberts,  I.  P.,  and  Wing,  H.  H.,  On  the  Deterioration  of  Farmyard 
Manure  by  Leaching  and  Fermentation;  Cornell  Agr.  Exp.  Sta.,  Bui.  13, 
1889.  Also,  Roberts,  I.  P.,  The  Production  and  Care  of  Farm  Manure; 
Cornell  Agr.  Exp.  Sta.,  Bui.  27,  1891.  Also,  Watson,  G.  C,  The  Produc- 
tion of  Manure;  Cornell  Agr.  Exp.  Sta.,  Bui.  56,  1893. 

'Thorne,  C.  E.,  Farm  Manures,  p.  97;  New  York,  1914. 

'Thorne,  C.  E.,  and  others,  The  Maintenance  of  Fertility;  Ohio  Agr. 
Exp.  Sta.,  Bui.  183,  1907. 

4  Watson,  G.  C,  The  Production  of  Manure;  Cornell  Agr.  Exp.  Sta., 
Bui.  56,  1893.  6 

6  Van  Slyke,  L.  L.,  Fertilisers  and  Crops,  p.  294;  New  York,  1912. 

8  Hart,  E.  B.,  and  Tottingham,  W.  E.,  General  Agricultural  Chemistry, 
p.  125;  Madison,  Wis.,  1913. 


FARM  MANURE  515 

Average  manure  (bedding  plus  excrement)  is  about  75  per 
cent,  water.  This  means  that  from  100  pounds  of  mixed  food 
there  results  50  pounds  of  manurial  dry  matter,  25  pounds 
of  litter,  and  225  pounds  of  water  or  300  pounds  in  all.  The 
weight  of  the  food  consumed  multiplied  by  three  should  give 
in  a  rough  way  the  weight  of  the  fresh  excrement  plus  its 
litter. 

292.  Loss  of  crop  constituents  in  the  production  and 
handling  of  manure. — Any  system  of  agriculture,  whether  it 
be  grain  farming,  animal  husbandry,  or  some  specialized  type 
such,  as  trucking,  must  ultimately  arrange  for  the  addition 
of  certain  nutrients  to  replace  those  lost  in  the  crop,  in  drain- 
age and  through  biological  activity.  It  is  evident,  however, 
that  even  if  all  of  the  crop  constituents  were  returned  to  the 
soil,  a  constant  degree  of  fertility  would  not  be  maintained, 
although  the  organic  matter  and  possibly  the  nitrogen,  if 
legumes  were  included  in  the  rotation,  might  not  greatly  de- 
crease. The  large  loss  of  certain  nutrients  in  the  drainage 
water  must  always  be  considered  in  any  rational  system  of 
soil  fertility. 

Since  farm  manure  lessens  or  even  eliminates  the  need  of  a 
green-manure  and  at  the  same  time  offers  a  means  of  lower- 
ing the  fertilizer  bill,  it  is  worth  while  to  inquire  what  pro- 
portion of  the  nutrients  contained  in  the  crop  may  be  re- 
turned to  the  soil  in  the  resulting  manure.  The  losses  en- 
tailed are  three:  (1)  those  that  occur  in  the  handling  and 
feeding  of  the  crop,  (2)  those  incurred  as  the  food  passes 
through  the  animal,  and  (3)  those  due  to  the  handling  and 
storage  of  the  manure  produced. 

293.  Losses  during  manurial  production. — A  certain 
amount  of  every  crop  is  lost  before  it  is  finally  consumed  by 
the  animal.  Such  loss,  while  important,  is  usually  small  on 
every  farm,  especially  when  compared  to  the  nutrients  re- 
tained by  the  animal.  Attention  is,  therefore,  particularly 
directed  towards  those  losses  sustained  by  the  food  as  it  un- 


516        NATURE  AND  PROPERTIES  OF  SOILS 

dergoes  normal  digestion.    Some  of  the  data  available  in  this 
respect  are  quoted  below: 

Table  CXII 

PERCENTAGE   OF    ORIGINAL    FOOD    CONSTITUENTS    RECOVERED 
IN  FRESH  MANURE. 


Animal 

Steers,  Ohio  x   

Steers,  Penn.2 

Steers,  England  3 

Milking  cows,  Illinois4. 
Milking  cows,  Penn. 5  . . 
Milking  cows,  England  6 
Heifers,  England  7  . . . . 
Sheep,  Ohio  8 


NH, 


61.0 
69.4 
95.5 
80.3 
84.6 
71.8 
77.8 
68.0 


P2o5 


86.8 
75.1 
93.0 
73.3. 
70.7 
75.0 
78.4 
87.0 


K3o 


82.4 
81.2 
98.5 
76.0 
91.0 
90.0 
86.4 
91.5 


As  might  be  expected,  the  data  are  quite  variable,  depend- 
ing on  the  age,  condition,  individuality  and  class  of  animal, 
and  the  character  of  the  food.  As  a  generalization  and  for 
purposes  of  calculation,  it  may  be  considered  that  three- 
fourths  of  the  ammonia,  four-fifths  of  the  phosphorus,  nine- 
tenths  of  the  potash,  and  one-half  of  the  organic  matter  are 
recovered  in  the  manure.9    This  means  losses  of  about  25,  2Q, 

1  Thome,  C.  E.,  Maintenance  of  Fertility:  Ohio  Agr.  Exp.  Sta,  Bui. 
183,  p.  200,  1907. 

3Frear,  W.,  Losses  of  Manure;  Pa.  Agr.  Exp.  Sta.,  Bui.  63:  Apr. 
1903.  ' 

3  Hall,  A.  D.,  Fertilisers  and  Manures,  p.  180;  New  York,  1921. 

4  Hopkins,  C.  G.,  Soil  Fertility  and  Permanent  Agriculture,  p.  201, 
Boston,  1910. 

6Sweetser,  W.  S.,  The  Manurial  Value  of  the  Excreta  of  Milch  Cows; 
Pa.  State  Coll.,  Ann.  Rep.,  1899-1900,  j>p.  321-351. 

6  Hall,  A.  D.,  Fertilisers  and  Manures,  p.  180 ;  New  York,  1921. 

7 Wood,  T.  B.,  Losses  in  Making  and  Storing  Farm  Yard  Manure; 
Jour.  Agr.  Sci.,  Vol.  II,    pp.  207-215,  1907-08. 

8Thorne,  C.  E.,  Maintenance  of  Fertility;  Ohio  Agr.  Exp.  Sta.,  Bui. 
183,  p.  202,  1907.  *  *  ' 

•See  Hopkins,  C.  G.,  Soil  Fertility  and  Permanent  Agriculture,  p.  206; 
Boston,   1910.  *  ' 

Also,   Pippin,   E.   O.,   Live  Stock  and   the  Maintenance   of   Organic 


FARM  MANURE  517 

10  and  50  per  cent.,  respectively,  for  these  constituents.  While 
such  losses  are  necessary  and  are  usually  compensated  by  the 
animal  products,  their  magnitude  must  be  considered  in  esti- 
mating- the  value  of  manure  in  the  ordinary  rotation. 

294.  Losses  due  to  handling  and  storage. — As  about  one- 
half  of  the  ammonia  and  three-fifths  of  the  potash  of  average 
farm  manure  are  in  a  soluble  condition,  the  possibility  of  loss 
by  leaching  is  usually  great,  even  though  the  manure  is  not 
exposed  to  especially  heavy  rainfall.  The  loss  of  phosphorus 
is  also  of  some  consequence.  In  addition,  decomposition,  espe- 
cially that  of  an  aerobic  nature,  will  cause  a  rapid  waste  of 
ammonia,  one-half  of  that  present  being  especially  susceptible. 
Packing  and  moistening  the  manure  will  change  the  decay 
from  aerobic  to  anaerobic,  thus  reducing  the  waste  of  am- 
monia while  encouraging  the  simplification  of  the  manurial 
constituents.  Tight  floors  in  the  stables  and  impervious  bot- 
toms in  the  manure  pit  or  under  the  manure  pile  will  con- 
siderably diminish  leaching  losses. 

It  is  impossible,  in  quoting  figures  for  waste  of  manure, 
to  separate  the  losses  due  to  fermentation  and  putrefaction 
from  those  due  to  leaching.  The  two  processes  go  on  simul- 
taneously, the  loss  from  one  source  being  dependent,  to  a  cer- 
tain extent,  on  the  other.  It  is  only  the  nitrogen,  however, 
that  may  be  lost  by  both  decomposition  and  leaching,  the  min- 
erals being  wasted  only  through  the  latter  avenue. 

While  the  figures  are  variable  (Table  CXIII),  it  is  easily 
seen  that  one-half  of  the  ammonia  and  potash  and  one-third  of 
the  phosphoric  acid  are  readily  lost  under  fairly  careful  meth- 
ods of  storage.  On  the  average  farm  where  manure  very  often 
remains  outside  for  several  months,  the  losses  will  run  much 
higher,  easily  amounting  to  50  per  cent,  of  the  organic  mat- 
ter, 60  per  cent,  of  the  ammonia,  40  per  cent,  of  the  phos- 

Matter  in  the  Soil;  Jour.  Amer.  Soc.  Agron.,  Vol.  9,  No.  3,  pp.  97-105, 
Mar.  1917. 

Also  Armsby,  H.  P.,  and  Fries,  J.  A.,  Net  Energy  Values  of  Feeding 
Stuffs  for  Cattle;  Jour.  Agr.  Res.,  Vol.  Ill,  pp.  435-491,  1915. 


518        NATURE  AND  PROPERTIES  OF  SOILS 

phoric  acid,  and  65  per  cent,  of  the  potash.  This  means  a  loss 
of  at  least  one-half  of  the  nutrient  constituents  of  the  ma- 
nure and  considerably  over  one-half  of  the  fertilizing  value, 
since  the  elements  wasted  are  those  most  readily  available  to 
plants.  Considering  the  losses  which  the  food  sustains  during 
digestion  and  the  waste  of  the  manure  in  handling  and  stor- 
age, it  cannot  be  expected  that  more  than  25  per  cent,  of  the 

Table  CXIII 

LOSSES  FROM   MANURE  THROUGH  LEACHING  AND 
FERMENTATION. 


Kind  of  Manure 

Horse  ■ 

HORSE  ■ 

Horse  2 

Cow1 

Cow3 

Steer  4 

Days  exposed .... 
Percentage  loss  of 

ammonia 

Percentage  loss  of 

phosphoric  acid 
Percentage  loss  of 

potash  

183 
36 
50 
60 

183 

60 
47 
76 

274 
40 
16 
34 

183 
41 
19 

8 

77 
31 
19 
43 

91 

30 
23 

58 

organic  matter,  30  per  cent,  of  the  ammonia,  50  per  cent,  of 
the  phosphoric  acid,  and  30  per  cent,  of  the  potash  of  the 
original  crop  will  reach  the  land.5     Even  if  leaching  losses 

1  Roberts,  I.  P.,  and  Wing,  H.  H.,  On  the  Deterioration  of  Farmyard 
Manure  by  Leaching  and  Fermentation;  Cornell  Agr.  Exp.  Sta.,  Bui.  13, 
1889. 

aSehutt,  M.  A.,  Barnyard  Manure.  Canadian  Dept.  Agr.,  Centr.  Exp. 
Farms,  Bui.  31,  1898. 

3  Thome,  C.  E.,  Farm  Manures,  p.  146 ;  New  York,  1914. 

4  Thorne,  C.  E.,  and  others,  The  Maintenance  of  Fertility;  Ohio  Agr. 
Exp.  Sta.,  Bui.  183,  1907. 

5Voeleker  and  Hall  have  drawn  up  recommendations  for  the  compen- 
sation of  the  out-going  English  tenant  for  manure  produced  on  the  farm 
but  not  realized  on.  They  suggest  that  he  receive  pay  at  fertilizer  prices 
for  one-half  of  the  nitrogen,  three-fourths  of  the  phosphoric  acid,  and 
all  of  the  potash  contained  in  the  food  consumed  during  the  last  year 
of  tenancy.  For  the  second,  third,  and  fourth  years  previous,  the  com- 
pensation value  shall  be  one-half  that  of  the  year  immediately  preced- 
ing. Voelcker,  A.,  and  Hall,  A.  D.,  The  Valuation  of  Unexhausted 
Manures;  Jour.  Eoy.  Agr.  Soc.  Eng.,  Vol.  63,  pp.  76-114,  1902. 


FARM  MANURE 


519 


OR.GA/V/C 
MATTER 


*"£         £<%■       «zo 


E£T/I/A/£D  3YA/V/MAL 


Lost  //v  /-/a/vdjl//vg 
amd  ^to/sage 


ADDED  TO  THE  SO/ L. 


Fig.  62. — Diagram  showing  the  proportion  of  the  important  constituents 
of  the  food  retained  by  the  animal,  lost  in  the  handling  and  the 
storage  of  the  manure  and  applied  to  the  soil  under  ordinary 
conditions. 


were  not  important,  a  self-sustaining  system  of  agriculture 
could  not  be  established  by  the  use  of  farm  manure  alone,  as 
organic  matter  is  the  only  constituent  that  would  be  added 
to  the  soil  in  amounts  that  approach  the  magnitude  of  the 
loss.1 

1  Fippin,  E.  O.,  Live  Stock  and  the  Maintenance  of  Organic  Mat- 
ter in  the  Soil;  Jour.  Amer.  Soc.  Agron.,  Vol.  9,  No.  3,  pp.  97-105, 
Mar.  1917. 


520        NATURE  AND  PROPERTIES  OF  SOILS 

295.  Two  phases  of  manurial  practice. — A  commercial 
fertilizer,  if  made  properly,  may  be  kept  for  long  periods 
unimpaired  and  is  always  in  a  condition  for  instant  applica- 
tion to  the  soil.  The  only  problem  confronting  the  farmer  is 
the  profitable  application  of  such  material.  Storage  is  a 
minor  factor.  Farm  manure,  on  the  other  hand,  although  a 
true  fertilizer,  presents,  because  of  its  peculiar  characteris- 
tics, serious  complications.  As  it  is  subject  to  tremendous 
losses  by  leaching,  putrefaction,  and  fermentation,  its  han- 
dling and  storage,  if  the  latter  becomes  necessary,  is  as  im- 
portant as  its  rational  utilization  on  the  land.  Manurial  prac- 
tice, therefore,  is  logically  discussed  under  two  headings: 
(1)  handling  and  storage,  and  (2)  utilization  of  the  manure 
in  the  field. 

296.  Care  of  manure  in  the  stalls. — Considerable  loss  to 
manure  occurs  in  the  stable,  due  to  decomposition  and  leach- 
ing. Before  the  urine  can  be  absorbed  by  the  litter,  it  is 
likely  to  decay  and  leach  away  in  considerable  amounts. 
Therefore,  the  first  care  is  to  the  bedding,  which  should  be 
chosen  for  its  absorptive  properties,  its  cost,  and  its  cleanli- 
ness. The  following  table  *  shows  the  approximate  absorptive 
capacity  of  some  common  litters.    (Table  CXIV,  page  521.) 

The  amount  of  litter  to  be  used  is  determined  by  the  char- 
acter of  the  food.  If  the  food  is  watery,  the  bedding  should 
be  increased.  In  general,  the  litter  amounts  to  about  one- 
fourth  of  the  dry  matter  of  the  food  consumed.  Sheep  re- 
quire about  a  pound  of  bedding  a  head,  cattle  from  eight  to 
ten  pounds,  and  horses  from  ten  to  fifteen  pounds.  No  more 
litter  than  is  necessary  to  keep  the  animal  clean  and  to  ab- 
sorb the  liquid  manure  should  be  used,  as  the  excrement  is 

1Beal,  W.  H.,  Barnyard  Manure;  U.  S.  Dept.  Agr.,  Farmers'  Bui. 
192,  1904.  *        8    ' 

Whisenand,  J.  W.,  Water-holding  Capacities  of  Bedding  Materials 
for  Live  Stock,  Amounts  Required  to  Bed  Animals,  and  Amounts  of 
Manure  Saved  by  Their  Use;  Jour.  Agr.  Res.,  Vol.  XIV,  No.  4,  pp. 
187-190,  July  1918. 


FARM  MANURE  521 

Table  CXIV 

ABSORPTIVE   POWER    OF    BEDDING   FOR   WATER. 


Material 

Percentage  of 
Water  Betained 

Mixed  shavings 

124 

Mixed  sawdust 

160 

Fine  pine  shavings 

185 

Muck 

200 

Wheat  straw 

210 

Oats  straw 

250 

Peat 

600 

Peat  moss 

1300 

thus  diluted  unnecessarily  with  material  which  often  does 
not  carry  large  quantities  of  available  fertilizing  ingredients. 

The  next  care  is  that  floors  should  be  tight,  so  that  the 
free  liquid  cannot  drain  away  but  will  be  held  in  contact  with 
the  absorbing  materials.  The  preserving  of  manures  in  stalls 
with  tight  floors  has  been  for  years  a  common  method  of  han- 
dling dung  in  England.  The  trampling  of  the  animals,  and 
the  continued  addition  of  litter  as  the  manure  accumulates, 
explain  the  reason  for  the  success  of  the  method.  The  follow- 
ing data,  from  Ohio,1  show  the  relative  recovery  of  food  ele- 
ments in  manure  produced  on  a  cement  floor  and  on  an  earth 
floor,  respectively.  The  experiment  was  conducted  with  steers 
over  a  period  of  six  months.  Even  with  a  good  dirt  floor,  the 
leaching  losses  are  considerable.     (Table  CXV,  page  522.) 

297.    Hauling  directly  to  the  field.2 — Where  it  is  possible 

1  Thome,  C.  E.,  Maintenance  of  Fertility;  Ohio  Agr.  Exp.  Sta.,  Bui. 
183,  p.  199,  1907. 

2  Good  discussions  of  handling  farm  manure  are  as  follows : 

Hart,  E.  B.,  Getting  the  Most  Profit  from  Farm  Manure;  Wis.  Agr. 
Exp.  Sta.,  Bui.  221,  1912. 

Beal,  W.  EL,  Barnyard  Manure;  U.  S.  Dept.  Agr.,  Farmers'  Bui.  192, 
1904. 

Eoberts,  I.  P.,  The  Fertility  of  the  Land,  Chapter  IX,  pp.  188-213; 
New  York,  1904. 


522 


NATURE  AND  PROPERTIES  OF  SOILS 


Table  CXV 

RECOVERY  OP  FOOD  ELEMENTS  IN  MANURE  PRODUCED  ON  CEMENT 
FLOOR;   ON  EARTH  FLOOR. 


Constituents 

Percentage  Recovery 

Cement  Floor 

Earth  Floor 

Ammonia 

74.7 
77.5 

87.8 

62.4 

Phosphoric  acid 

78.9 

Potash 

78.4 

Average 

.    80.0 

73.2 

to  haul  directly  to  the  field,  this  practice  is  to  be  advised, 
since  opportunities  for  excessive  losses  by  leaching  and  fer- 
mentation are  thereby  prevented.  Manure  may  even  be 
spread  on  frozen  ground  or  on  the  top  of  snow,  provided  the 
land  is  fairly  level  and  the  snow  is  not  too  deep.  This  sys- 
tem saves  time  and  labor,  and  when  leaching  does  occur  the 
soluble  portions  of  the  manure  are  carried  directly  into  the 
soil.  The  practice  of  allowing  the  manure  so  spread  to  lie 
on  the  surface  of  the  land  all  winter  is  sometimes  questioned, 
especially  in  New  England.1  On  sandy  soils  it  may  some- 
times be  better  practice  to  store  the  manure  until  spring. 

298.  Piles  outside. — Very  often  it  is  necessary  to  store 
manure  outside,  fully  exposed  to  the  weather.  When  this  is 
the  case,  certain  precautions  must  be  observed.  In  the  first 
place,  the  pile  should  be  located  on  level  ground  far  enough 
from  any  building  that  it  receives  no  extra  water  in  times 
of  storm.  The  sides  of  the  heap  should  be  steep  enough  to 
shed  water  readily,  while  the  depth  of  the  pile  should  be  such 
as  to  allow  little  leaching  even  after  heavy  storms.  The  earth 
under  the  manure  may  be  slightly  dished  in  order  to  prevent 


brooks,  W.  P.,  Methods  of  Applying  Manure: 
Bui.  196,  Sept.  1920. 


Mass.  Agr.  Exp.  Sta., 


FARM  MANURE  523 

loss  of  excess  water.  If  possible,  the  soil  of  the  depression 
should  be  puddled,  or,  better,  lined  with  cement. 

The  manure  should  be  kept  moist  in  dry  weather  in  order 
to  decrease  aerobic  action.  Each  addition  of  manure  should 
be  packed  in  place,  the  fresh  on  and  above  the  older.  This 
allows  the  gases  from  the  well-rotted  dung  to  pervade  the 
fresher  and  looser  portions,  thus  quickly  establishing  the 
anaerobic  conditions  so  essential  to  economic  and  favorable 
fermentation. 

Placing  fresh  manure  in  small  heaps  in  the  field  to  be 
spread  later,  is,  in  the  first  place,  poor  economy  of  labor. 
Moreover,  it  encourages  loss  by  decay,  while  at  the  same  time 
the  soluble  portions  of  the  pile  escape  into  the  soil  imme- 
diately underneath.  There  is  thus  a  poor  distribution  of  the 
essential  elements  of  the  dung,  and  when  the  manure  is  finally 
spread,  an  over-feeding  of  plants  at  one  point  and  an  under- 
feeding at  another  results.  A  low  efficiency  of  the  manure 
is  thus  realized.  This  method  of  handling  manure  is  not  to 
be  recommended. 

299.  Manure  pits. — Some  farmers,  especially  if  the 
amount  of  manure  produced  is  large,  find  it  profitable  to  con- 
struct manure  pits  of  concrete.  These  pits  are  usually  rec- 
tangular in  shape  with  a  shed  covering.  Often  one  or  even 
both  ends  are  open  to  facilitate  the  removal  of  the  manure. 
In  such  a  structure,  leaching  is  prevented  by  the  solid  bottom 
while  the  roof  allows  a  better  control  of  moisture  conditions. 
By  keeping  the  manure  carefully  spread  and  well  moistened, 
putrefaction  may  proceed  with  a  minimum  loss  of  nitrogen. 
Some  European  dairymen  even  go  so  far  as  to  utilize  a  cis- 
tern, into  which  is  shoveled  both  the  liquid  and  the  solid 
manure.  Later  when  decomposition  has  proceeded  suffi- 
ciently, the  material  is  pumped  out  and  applied  to  the  land. 
This  method  is  not  to  be  advocated  in  this  country  except 
under  special  conditions,  owing  to  the  cost  of  handling. 

300.  Covered  yards. — Another  method  of  storage  is  by 


524        NATURE  AND  PROPERTIES  OF  SOILS 

means  of  a  covered  barnyard.  Such  a  yard  should  have  a 
more  or  less  impervious  floor.  The  manure  is  spread  out  in 
the  yard  and  is  kept  thoroughly  packed  as  well  as  damp  by 
the  animals.  This  is  a  common  method  of  handling  the  ma- 
nure in  the  fattening  of  steers  in  the  Middle  West  and  pro- 
duces manure  at  a  minimum  loss,  providing  hogs  are  not  al- 
lowed to  follow  the  steers.  The  storage  of  manure  in  deep 
stalls,  a  favorite  method  in  England,  is  similar  to  this  system 
and  has  been  shown  to  be  very  economical.  It  also  affords  an 
opportunity  for  the  mixing  of  the  manure  from  different 
classes  of  animals.  The  desirability  of  this  has  already  been 
shown  in  the  case  of  horse  and  cow  excrements.  The  advan- 
tages of  trampling,  so  far  as  the  keeping  qualities  of  manure 
are  concerned,  are  clearly  shown  by  the  following  figures 
taken  from  the  work  of  Frear : * 


Table  CXVI 

LOSS   OF    MANURE   IN    COVERED    SHEDS. 


Condition 

Percentage  Loss  of 

NH3 

K20 

PA 

Covered  and  tramped 

5.7 
34.1 

5.5 

19.8 

8.5 

Covered  and  untramped 

14.2 

Throwing  manure  in  heaps  under  a  shed  and  allowing  hogs 
to  work  the  mass  over,  is  a  desirable  practice  so  far  as  food 
utilization  is  concerned.  It  interferes,  however,  with  a  proper 
and  economical  packing  of  the  manure.  The  question  to  be 
decided  is  whether  the  added  food  value  of  the  manure  over- 
balances the  extra  losses  by  decomposition  incurred  by  the 
rooting  of  the  swine. 

301.  Increased  value  of  protected  manure. — From  the 
previous  discussion,  it  is  evident  that  a  well-protected  and 

1  Frear,  W.,  Losses  of  Manure;  Pa.  Agr.  Exp.  Sta.,  Bui.  63,  1903. 


FARM  MANURE 


525 


carefully  preserved  manure  will  be  higher  in  available  plant 
constituents  than  one  not  so  handled.  Moreover,  the  agricul- 
tural value  of  such  manure  will  be  higher.  This  is  shown 
by  actual  tests  from  Ohio.1  Over  a  period  of  fourteen  years, 
in  a  three-years'  rotation  of  maize,  wheat,  and  hay,  a  stall 
manure  gave  a  yield  38  per  cent,  higher  than  that  with  a  yard 
manure. 

Table  CXVII 

INCREASE  YIELDS  FROM  YARD  AND  STALL   MANURE. 


Manure 

Average  Annual  Increase  to 
the  Acre 

Maize 
14  Crops 

Wheat 
14  Crops 

Clover 
11  Crops 

Yard,  8  tons  to  the  rota- 
tion  

18.6  bus. 
23.6  bus. 

26.8% 

9.5  bus. 
10.9  bus. 

14.7% 

801  lbs. 

Stall,  8  tons  to  the  rota- 
tion   

1395  lbs. 

Increase,  stall  over  yard 
manure 

74.1% 

In  New  Jersey,  fresh  manure  showed  a  gain  in  crop  yield 
53  per  cent,  higher  than  leached  manure  over  the  three  years 
immediately  following  the  application.  Such  figures  are 
worthy  of  careful  consideration. 

302.  Application  of  manure. — In  the  application  of  ma- 
nure to  the  land,  the  same  general  principles  observed  in  the 
use  of  any  fertilizer  should  be  kept  in  mind.  Of  these,  fine- 
ness of  division  and  evenness  of  distribution  are  of  prime  im- 
portance. The  efficiency  of  the  manure  may  be  raised  con- 
siderably thereby.     Moreover,  it  is  generally  better,  since  the 

1  Thome,  C.  E.,  and  others,  Plans  and  Summary  Tables  of  the  Experi- 
ments at  the  Central  Farm;  Ohio  Agr.  Exp.  Sta.,  Circ.  120,  p.  112, 
1912. 


526        NATURE  AND  PROPERTIES  OF  SOILS 

supply  of  manure  is  usually  limited  in  diversified  farming,  to 
decrease  the  amounts  at  each  spreading  and  cover  a  greater 
acreage.  Thus,  instead  of  adding  20  tons  to  the  acre,  10  tons 
may  be  applied  and  twice  the  area  covered.  Applications 
could  then  be  made  oftener  and  a  larger  and  quicker  net 
return  realized  for  each  ton  of  manure.  With  manure,  as 
with  any  fertilizer,  the  yield  to  the  acre  is  not  so  important 
as  the  crop  increase  for  a  given  increment  of  manure  added. 
The  influence  of  rate  of  application  on  increased  yield  to  a 
ton  of  manure  is  shown  bj"  the  Ohio  x  experiments  over  eight- 
een years  in  a  three-year  rotation  of  wheat,  clover  and  pota- 
toes, the  manure  being  placed  on  the  wheat. 


Table  CXVIII 

INCREASED    YIELD    TO    THE    TON    WHEN    MANURE    IS    APPLIED    IN 
DIFFERENT  AMOUNTS.     OHIO  EXPERIMENT  STATION. 


Rate 

Wheat 
(bus.) 

Clover 
(lbs.) 

Potatoes 
(bus.) 

4  tons  to  the  acre 

8  tons  to  the  acre 

16  tons  to  the  acre 

1.34 

.94 
.70 

177 

150 

99 

3.81 

2.79 
2.76 

Not  only  is  the  increased  efficiency  from  the  smaller  appli- 
cation apparent,  but  a  greater  recovery  of  the  manurial  fer- 
tility in  the  crops  also  results.  The  Ohio  experiments  show 
that  in  the  first  rotation  after  the  manure  is  applied,  a  25  to 
30  per  cent,  higher  recovery  may  be  expected  from  the  8  tons 
treatment  than  from  the  16  tons. 

Evenness  of  application  and  fineness  of  division  are  greatly 
facilitated  by  the  use  of  a  manure-spreader.  This  also  makes 
possible  the  uniform  application  of  small  amounts  of  manure, 

1  Thorne,  C.  E.,  and  others,  Plans  and  Summary  Tables  of  the  Experi- 
ments at  the  Central  Farm;  Ohio  Agr.  Exp.  Sta.,  Circ.  120,  p.  108, 
1912. 


FARM  MANURE  527 

even  as  low  as  5  or  6  tons  to  the  acre.  It  is  impossible  to 
spread  so  small  an  amount  by  hand  and  obtain  an  even  dis- 
tribution. Moreover,  a  spreader  lessens  the  labor  and  more 
than  doubles  the  amount  of  manure  one  man  can  apply  a  day. 
When  any  considerable  quantity  of  manure  is  to  be  handled, 
a  manure-spreader  will  pay  for  itself  in  a  season  or  two  at  the 
most. 

Whether  manure  should  be  plowed  under  or  not  depends 
largely  on  the  crop  on  which  it  is  used.  On  timothy  it  is 
spread  as  a  top  dressing.  Ordinarily,  however,  it  is  plowed 
under.  This  is  particularly  necessary  if  the  manure  is  long, 
coarse,  and  not  well-rotted.  It  should  not  be  turned  under 
so  deep,  however,  as  to  prevent  ready  decay.  If  manure  is 
fine  and  well  decomposed,  it  may  be  harrowed  into  the  surface 
soil.  The  method  employed  depends  on  the  crop,  the  soil,  and 
the  condition  of  the  manure.  The  amount  to  be  applied  va- 
ries considerably.  Eight  tons  to  the  acre  would  be  a  light 
dressing,  15  tons  a  medium  dressing,  and  25  tons  heavy  for 
an  ordinary  soil.  In  trucking  land,  however,  as  high  as  50 
or  100  tons  are  often  used. 

303.  Reinforcement  of  manure. — The  reinforcement  of 
farm  manure  is  designed  to  accomplish  two  things  in  the  han- 
dling of  this  product:  (1)  checking  loss  due  to  leaching  and 
decomposition,  and  (2)  balancing  the  manure  and  rendering 
its  agricultural  value  higher.  Four  chemicals  may  be  used 
in  this  reinforcement:  gypsum  (CaSOJ,  kainit  (mostly 
K2S04),  acid  phosphate  (CaH4(POJ2  +  CaSOJ,  and  floats 
(raw  rock  phosphate,  Ca3(P04)2). 

Gypsum  and  kainit  are  supposed  to  react  with  the  ammonia 
of  the  manure,  changing  it  to  ammonium  sulfate,  a  stable 
compound.  As  gypsum  is  rather  insoluble,  its  action  is  prob- 
ably slow.  It  may  be  applied  either  in  the  stable  or  on  the 
manure  pile,  usually  at  the  rate  of  100  pounds  to  the  ton. 
It  has  no  balancing  effect.  Kainit  is  soluble  and  because  of 
its  caustic  tendencies  should  not  come  into  contact  with  the 


528        NATURE  AND  PROPERTIES  OF  SOILS 

feet  of  the  animals.  It  must  not  be  spread  on  the  manure 
until  the  stock  are  out  of  the  way.  Since  manure  is  unbal- 
anced as  to  phosphorus,  the  agricultural  value  of  kainit  is 
slight.  When  applied,  it  is  generally  used  at  the  rate  of  50 
pounds  to  the  ton  of  manure. 

Acid  phosphate  is  partially  soluble  and  will  not  only  react 
readily  with  the  ammonia  but  will  tend  to  raise  the  phos- 
phorus content  to  the  proper  point.  From  40  to  80  pounds 
of  acid  phosphate  are  generally  recommended  to  a  ton  of 
average  farm  manure.  It  should  not  be  allowed  to  come  into 
contact  with  the  feet  of  farm  animals. 

Raw  rock  phosphate,  or  floats,  is  a  very  insoluble  compound, 
and  consequently  reacts  but  slowly  with  the  soluble  constitu- 
ents of  manure.  Carrying  such  a  large  percentage  of  phos- 
phorus, it  tends  to  balance  the  manure  and  to  raise  its  agri- 
cultural value.  It  is  supposed  that  the  intimate  relationship 
between  the  phosphate  and  the  decaying  manure  increases  the 
availability  of  the  former  to  plants  when  the  mixture  is  added 
to  the  soil.  The  reinforcement  is  usually  at  the  rate  of  75 
to  100  pounds  to  a  ton  of  manure. 

Experimental  data  have  shown  that  these  various  rein- 
forcements have  no  particular  effect  on  the  nature,  function, 
and  number  of  the  bacterial  flora.  Their  conserving  influ- 
ence, if  any,  when  the  manure  is  exposed,  might  be  in  check- 
ing leaching  and  in  preventing  loss  of  ammonia.  The  follow- 
ing figures  from  Ohio  experiments  x  show  how  slight  this  con- 
serving effect  is.  The  reinforcement  was  at  the  rate  of  40 
pounds  to  the  ton.     ( See  Table  CXIX,  page  529. ) 

It  is  immediately  evident  that  kainit  and  gypsum  do  not 
conserve  the  manure,  and,  although  acid  phosphate  and  floats 
show  some  influence,  it  is  slight  and  evidently  well  within  the 
experimental  error.  The  principal  benefit  from  reinforcing 
manure,  if  any,  must,  therefore,  be  as  a  balancing  agent.    The 

1  Thome,  C.  E.,  and  others,  The  Maintenance  of  Fertility:  Ohio  Act. 
Exp.  Sta.,  Bui.  183,  p.  206,  1907. 


FARM  MANURE 


529 


Table  CXIX 

CONSERVING   EFFECT    OF   REINFORCING   AGENTS   ON    MANURE 
EXPOSED  FOR  THREE  MONTHS. 


Treatments 

Eatio  Values  of  a 
Ton  of  Manure 

Percentage 

IN  JANUARY 

IN   APRIL 

No  reinforcement 

100 

93 
102 
128 
106 

64 
67 
66 
93 
75 

36 

With  gypsum 

38 

With  kainit 

35 

With  floats 

27 

With  acid  phosphate 

29 

figures  from  Ohio  1  over  a  period  of  fourteen  years  in  a  rota- 
tion of  maize,  wheat,  and  hay  may  be  taken  as  evidence  re- 
garding this  point.  The  manure  treated  and  handled  as  above 
was  added  to  the  maize  at  the  rate  of  8  tons  to  the  acre. 

It  is  evident  that  the  principal  benefit  of  reinforcing  ma- 
nure lies  in  the  balancing  influence  and  that  acid  phosphate 
and  floats  are  the  most  desirable  agents.     It  is  also  evident 


Table  CXX 

INFLUENCE  OF  REINFORCING   ON   THE  EFFECTIVENESS 
OF   MANURE. 


Average  Annual  Increase  to  the  Acre 

Ratio  Value 
of  Increase 

Treatment 

Corn 
14  Crops 

Wheat 
14  Crops 

Hay 
11  Crops 

Per  Ton  of 

Manure 

No   reinforcement.  . 

With  gypsum 

With  kainit 

With  floats 

With  acid  phosphate 

18.6  bus. 

23.6  bus. 

23.7  bus. 
25.0  bus. 
30.6  bus. 

9.5  bus. 
11.6  bus. 
11.3  bus. 
12.9  bus. 
15.1  bus. 

801  lbs. 

916  lbs. 
1156  lbs. 
1578  lbs. 
1853  lbs. 

100 
119 
115 
138 
161 

1  Thome,  C.  E.,  and  others,  Plans  and  Summary  Tables  of  the  Experi- 
ments at  the  Central  Farm;  Ohio  Agr.  Exp.  Sta.,  Circ.  120,  p.  112, 
1912. 


530        NATURE  AND  PROPERTIES  OF  SOILS 

that  floats,  if  added  in  money  values  equal  to  acid  phosphate, 
should  be  about  as  satisfactory  as  a  reinforcing  material. 

304.  Lime  and  manure. — Very  often  it  would  be  a  sav- 
ing of  labor  to  apply  lime  and  manure  to  the  soil  at  the  same 
time.  This  can  readily  be  done  with  the  carbonated  forms. 
Such  lime  may  be  mixed  with  the  manure,  either  in  the  stable 
or  in  the  pile,  without  any  danger  of  detrimental  results.  The 
close  union  of  the  lime  and  manure  may  increase  the  effective- 
ness of  the  former  and  at  the  same  time  promote  a  better  type 
of  decomposition  in  the  latter.  If  the  soil  is  really  in  need  of 
calcium,  however,  a  separate  application  of  lime  is  much  bet- 
ter, as  the  amount  of  calcium  added  with  the  manure  is  never 
large.  Caustic  compounds  of  lime  such  as  calcium  oxide 
(CaO)  and  calcium  hydroxide  (Ca(OH)2)  must  be  kept  from 
manure.  These  forms  readily  react  with  the  ammonium  car- 
bonate coming  from  the  urea,  and  cause  the  liberation  of 
ammonia,  which  may  be  readily  lost  to  the  air : 

CON2H4  +  2H20  =  (NHJ2C03 
(NH4)2C03  +  Ca(OH)2  =  CaC03  +  2NH4OH 

A  stable  or  shed  containing  manure  may  be  at  once  deodor- 
ized by  the  use  of  quicklime,  but  with  the  loss  of  much  nitro- 
gen. If  the  manure  is  to  be  worked  into  the  surface  soil,  the 
caustic  lime  may  be  applied  some  days  before  and  if  it  is  in 
thorough  contact  with  the  soil,  it  will  change  to  the  carbonate 
before  the  manure  is  added.  When  the  manure  is  plowed 
under,  the  lime  is  best  added  after  the  plowing  and  thor- 
oughly harrowed  in  as  the  seed-bed  is  prepared. 

305.  Manure  and  composting.— A  compost  is  usually 
made  up  of  alternate  layers  of  manure  and  some  vegetable 
matter  that  is  to  be  decayed.  Layers  of  sod  or  of  soil  high  in 
organic  matter  are  often  introduced.  The  manure  supplies 
the  decay  organisms  and  starts  biological  activities.  The 
foundation  of  such  a  compost  is  usually 'soil,  and  the  pile  is 
preferably   capped  with   earth.     The  mass  should  be   kept 


FARM  MANURE 


531 


moist  in  order  to  prevent  loss  of  ammonia  and  to  encourage 
vigorous  bacterial  action.  Acid  phosphate  or  raw  rock  phos- 
phate and  a  potash  fertilizer  are  often  added,  to  balance  up 
the  mixture  and  make  it  a  more  effective  fertilizer.  Lime  is 
also  introduced,  to  react  with  such  organic  acids  as  may  tend 
to  interfere  with  proper  decay.  Undecayed  plant  tissue, 
such  as  sod,  leaves,  weeds,  grass,  sticks,  or  organic  refuse  of 
any  kind,  may  thus  be  changed  slowly  to  a  form  which  will  be 
valuable  in  building  up  the  soil  and  in  nourishing  plants. 
Even  garbage  may  be  disposed  of  in  such  a  manner. 

306.  Residual  effects  of  manure. — No  other  fertilizer 
exerts  such  a  marked  residual  effect  as  does  farm  manure. 
As  it  is  applied  in  large  amounts,  its  physical  and  biological 
influences  are  of  necessity  very  great  and  persist  for  a  con- 
siderable time.  As  only  about  one-half  the  nutrients  of  farm 
manure  are  readily  available,  the  residual  effect  of  its  fertiliz- 
ing elements  carry  over  into  succeeding  years.  Hall x  pre- 
sents the  following  comparative  data  regarding  the  recovery 
of  nitrogen  from  various  fertilizers.  The  crop  used  was  man- 
golds. The  low  recovery  of  the  nitrogen  from  the  manure  is 
of  especial  note.  There  is  no  reason  to  believe  that  the  pot- 
ash of  the  manure  would  be  any  more  readily  available  and 
the  phosphoric  acid  would  certainly  show  a  lower  recovery. 

Table  CXXI 

RECOVERY  OF  NITROGEN  IN  A   CROP  OF   MANGOLDS. 


Sodium  nitrate .... 
Ammonium  salts .  . . 

Rape  cake 

Farm  manure 


Bate  to 
the  Acre 


550  lbs. 

400  lbs. 

2000  lbs. 

14  tons 


Yield  in 
Tons 


17.95 
15.12 
20.95 
17.44 


Percentage 

Eecovery  of 

Nitrogen 


78.1 
57.3 
70.9 
31.6 


1  Hall,  A.  D.,  Fertilizers  and  Manures,  p.  210;  New  York,  1921. 


532        NATURE  AND  PROPERTIES  OF  SOILS 

The  length  of  time  through  which  the  effects  of  an  appli- 
cation of  farm  manure  may  be  detected  in  crop  growth  is 
very  great.  Hall 1  cites  data  from  the  Rothamsted  Experi- 
ments in  which  the  effects  of  eight  yearly  applications  of  14 
tons  each  were  apparent  forty  years  after  the  last  treatment. 
This  is  an  extreme  case.  Ordinarily,  profitable  increases  may 
be  obtained  from  manure  only  from  two  to  five  years  after 
the  treatment.2  The  fact  remains,  nevertheless,  that  of  all 
fertilizers,  farm  manure  is  the  most  lasting  and  lends  the  most 
stability  to  the  soil. 

307.  The  place  of  manure  in  the  rotation.3— With 
trucking,  garden,  and  greenhouse  crops,  the  applications  of 
large  amounts  of  manure  year  after  year  have  proven  advis- 
able. As  a  matter  of  fact,  manure  has  shown  itself,  especially 
if  balanced  with  phosphoric  acid,  to  be  the  best  fertilizer  for 
intensive  operations.  This  is  due  not  only  to  the  nutrients 
carried  by  the  manure,  but  to  the  large  amounts  of  easily 
decomposed  organic  matter  that  are  at  the  same  time  intro- 
duced. In  a  rotation  involving  the  staple  crops,  such  as  maize, 
oats,  wheat,  hay,  and  the  like,  less  intensive  applications  are 
advisable,  not  only  because  of  a  lack  of  manure  but  because 
the  return  to  a  ton  of  manure  applied  must  be  raised  as  high 
as  possible.  On  the  average  farm,  there  is  less  than  one  ton 
of  manure  produced  to  an  acre  of  arable  land.  Moreover,  the 
return  from  manure  will  vary  according  to  its  place  in  the 
rotation.  This  has  proved  to  be  the  case  with  commercial 
fertilizers  and  the  fact  is  becoming  more  and  more  apparent 
with  farm  manure. 

In  general,  meadows  and  pastures  derive  more  benefit  from 
manure,  either  residually  or  directly,  than  any  other  crop. 

1Hall,  A.  D.,  Fertilizers  and  Manures,  p.  213;  New  York,  1921. 

aVoelcker,  A.,  and  Hall,  A.  D.,  The  Valuation  of  Unexhausted 
Manure  Obtained  by  the  Consumption  of  Foods  by  Stock:  London, 
1903. 

3  See  Thorne,  C.  E.,  Farm  Manures,  Chaps.  XI  and  XIII,  New  York, 
1914. 


FARM  MANURE 


533 


The  long  tests  conducted  by  the  Pennsylvania  and  Ohio  ex- 
periment stations  x  have  established  this  fact.  The  following 
data  from  Illinois  2  may  be  cited,  comparing  the  response  of 
maize  and  oats  when  manured  to  the  increased  yield  of  clover 
receiving  the  same  treatment.     (See  Table  CXXII,  page  534.) 


CROP 


FOOD  LOSSES 


OM    - 

B* 

NH5- 

P2O5- 

50» 

K20  - 

50  » 

Fig.  63. — Diagram  showing  the  proportion  of  the  harvested  crop  added  to 
the  soil  in  farm  manure  under  average  conditions. 


It  is  easy  to  see  that  a  liberal  dressing  of  manure  on  the 
hay  and  pasture  will  markedly  increase  the  crop.  Neverthe- 
less, as  manure  is  available  in  limited  amounts  on  the  average 
farm  and  as  commercial  fertilizers  will  give  almost  as  good 
returns  on  hay,  it  is  generally  considered  judicious,  except  in 

1Hunt,  T.  F.,  General  Fertilizer  Experiments;  Ann.  Eep.  Penn.  Agr. 
Exp.  Sta.,  1907-1908,  pp.  68-93. 

Thome,  C.  E.,  and  others,  Plans  and  Summary  Tables  of  the  Experi- 
ments at  the  Central  Farm;  Ohio  Agr.  Exp.  Sta.,  Circ.  120,  pp.  101- 
105,  1912. 

2  Hopkins,  C.  G.,  Thirty  Years  of  Crop  Rotation  in  Illinois;  111.  Agr. 
Exp.  Sta.,  Bui.  125,  p.  337,  1908. 


534        NATURE  AND  PROPERTIES  OF  SOILS 
Table  CXXII 

INFLUENCE  OF  MANURE  ON  MAIZE,  OATS,  AND  CLOVER. 


Average  Percentage 
Increase 

Katio  Value   op 
Increase 

Maize  and 
Oats 

Clover 

Maize  and 
Oats 

Clover 

Manure  alone. . 
Manure,  lime 
and  phosphate 

11 
30 

92 
141 

100 
162 

134 
206 

certain  cases,  to  reserve  most  of  the  manure  for  other  crops. 
The  top  dressing  of  meadows  is,  however,  always  an  allowable 
practice,  especially  on  new  seeding  or  on  hay  land  that  is 
soon  to  be  plowed  for  maize. 

As  a  food  producer,  maize  has  no  close  rival.  Where  the 
climate  is  favorable,  a  75-bushel  crop  of  maize  is  as  easily 
secured  as  40  bushels  of  wheat  or  300  bushels  of  potatoes  to 
the  acre.  Moreover,  the  maize  stover  may  be  made  more  valu- 
able as  roughage  than  the  straw  of  oats,  wheat,  or  rye.  The 
maize  plant  must  have,  however,  for  its  successful  growth 
plenty  of  available  nitrogen.  In  addition,  its  response  to 
abundant  organic  matter  indicates  the  utilization  of  certain 
organic  compounds.  These  considerations  argue  for  the  use 
of  most  of  the  farm  manure  on  the  maize  when  this  crop  is 
important,  especially  if  the  supply  of  manure  is  limited. 
Again  the  maize  crop  is  ready  for  the  manure  in  the  spring 
and  is  generally  grown  on  land  where  the  excreta  may  be 
distributed  during  the  previous  winter  and  fall. 

Potatoes  are  a  spring  crop  and  where  they  are  prominent 
in  the  rotation  may  receive  liberal  applications  of  manure. 
If  potatoes  are  the  money  crop,  this  should  by  all  means  be 
the  practice.  Oats,  because  of  the  tendency  to  lodge,  gener- 
ally follow  maize  or  potatoes  as  a  residual  feeder,  receiving,  if 
necessary,  a  dressing  of  commercial  fertilizer.    If  manure  is 


FARM  MANURE  535 

used  on  fall  wheat,  a  great  loss  of  manurial  value  is  incurred, 
due  to  the  necessity  of  storage  during  the  summer  months. 
Moreover,  commercial  fertilizers  high  in  phosphorus  are  so 
convenient  and  effective  on  wheat  that  the  use  of  manure  on 
this  crop  is  becoming  rather  uncommon,  although  manure 
may  be  used  to  advantage  as  a  fall  and  winter  dressing,  since 
it  not  only  stimulates  the  wheat  but  is  of  great  value  to  the 
new  seeding  as  well.  Where  cotton  and  tobacco  are  the  staple 
crops,  they  should  receive  at  least  a  part  of  the  manure  pro- 
duced. The  value  of  manure  in  orchards  should  not  be  over- 
looked, especially  on  sandy  soils.  The  up-keep  of  organic 
matter,  the  conservation  of  moisture,  and  the  nutrients  sup- 
plied are  as  important  here  as  in  any  phase  of  soil  manage- 
ment. 

308.  Resume. — Barnyard  manure,  from  the  standpoint 
of  soil  fertility,  is  the  most  valuable  by-product  of  the  farm. 
A  careful  farmer  will,  therefore,  attempt  to  utilize  it  in  the 
most  economical  way.  The  handling  of  manure  in  such  a 
manner  that  only  a  minimum  waste  occurs  from  the  time 
the  manure  is  voided  until  it  has  reached  the  land  is  not  an 
easy  problem.  Manure  is  so  susceptible  to  the  loss  of  valuable 
ingredients,  both  by  leaching  and  by  decay,  that  careful 
methods  must  be  employed.  Tight  floors  in  the  stable  and 
covered  sheds  or  manure  pits  are  always  advisable.  Hauling 
immediately  to  the  field  is  the  wisest  procedure,  yet  even  with 
the  best  of  care  more  than  50  per  cent,  of  the  fertilizing  value 
is  usually  lost.  The  problem  of  rational  manurial  utilization 
is  not  solved,  however,  by  careful  handling  and  storage  alone. 
Manure  must  be  applied  in  such  a  condition,  in  such  amounts 
and  at  such  a  point  in  the  rotation  as  to  realize  a  reasonable 
return  for  every  increment  applied.  The  reinforcement  of 
farm  manure  with  phosphoric  acid  is  by  no  means  an  unim- 
portant feature.  In  fact,  all  of  the  principles  which  are  ob- 
served in  the  profitable  utilization  of  commercial  fertilizers 
should  be  adhered  to  in  the  use  of  farm  manures. 


536        NATURE  AND  PROPERTIES  OF  SOILS 

A  permanent  system  of  agriculture  evidently  cannot  be 
established  by  merely  returning  all  the  manure  possible  to 
the  land,  as  approximately  only  25  per  cent,  of  the  organic 
matter,  30  per  cent,  of  the  ammonia,  50  per  cent,  of  the  phos- 
phoric acid,  and  30  per  cent,  of  the  potash  of  the  food  con- 
sumed on  the  farm  ever  reach  the  land  in  the  manure.  Never- 
theless, it  is  certainly  worth  the  while  of  a  farmer  to  use 
some  care  in  handling  this  product  and  some  thought  as  to 
its  rational  utilization  in  the  field.  Even  if  the  manure 
should  aid  only  in  the  up-keep  of  organic  matter,  the  effort 
would  be  worth  while.  Reasonable  care  in  the  handling  of 
farm  manure  will  save  this  country  thousands  of  pounds  of 
manurial  fertility  which  are  now  utterly  lost  and  at  the  same 
time  increase  by  thousands  of  dollars  the  food  production. 


CHAPTER  XXV 
GREEN-MANURES * 

From  time  immemorial  the  turning-under  of  a  green-crop 
to  supply  organic  matter  to  the  soil  has  been  a  common  agri- 
cultural practice.  Records  show  that  the  use  of  beans,  vetches, 
and  lupines  for  such  a  purpose  was  well  understood  by  the 
Romans,  who  probably  borrowed  the  practice  from  nations 
of  greater  originality.  The  art  was  lost  to  a  great  extent  dur- 
ing the  Middle  Ages,  but  was  revived  again  as  the  modern 
era  was  approached.  At  the  present  time,  green-manuring 
is  considered  a  part  of  a  well-established  system  of  soil  man- 
agement, and  is  given  a  place,  when  possible,  in  every  ra- 
tional plan  for  permanent  soil  improvement. 

309.  Importance  of  green-manures. — The  plowing  under 
of  some  succulent  rapid-growing  crop,  such  as  oats,  rye,  or 
clover,  tends  to  bring  about  three  desirable  soil  conditions; 
additional  organic  matter,  a  betterment  of  the  physical  con- 
dition of  the  soil,  and  a  rise  in  the  nitrogen  content  of  the 
land,  if  the  crop  is  an  inoculated  legume.     If  conditions  are 

1  Penny,  C.  L.,  Clover  Crops  as  Green  Manures;  Del.  Agr.  Exp.  Sta., 
Bui.  60,  1903. 

Storer,  F.  H.,  Agriculture,  pp.  137-175;  New  York,  1910. 

Lipman,  J.  G.,  Bacteria  in  Eelation  to  Country  Life,  Chapter  XXIV, 
pp.  237-263;  New  York,  1911. 

Piper,  C.  V.,  Leguminous  Crops  for  Green  Manuring;  U.  S.  Dept.  Agr., 
Farmers'  Bui.  278,  1907. 

Spillman,  W.  J.,  Renovation  of  Worn-out  Soils;  U.  S.  Dept.  Agr., 
Farmers '  Bui.  245,  1906. 

Pieters,  A.  J.,  Green  Manuring:  A  Review  of  tine  American  Experi- 
ment Station  Literature;  Jour.  Amer.  Soe.  Agron.,  Vol.  9,  No.  2,  pp. 
62-82,  Feb.  1917;  Vol.  9,  No.  3,  pp.  109-126,  Mar.  1917;  Vol.  9,  No.  4, 
pp.  162-190,  Apr.  1917. 

537 


538        NATURE  AND  PROPERTIES  OF  SOILS 

favorable,  an  increase  in  crop  production  should  result. 
Where  there  is  a  shortage  of  farm  manure,  the  practice  be- 
comes of  special  importance  since  roots  and  crop  residues  are 
usually  insufficient  to  maintain  the  organic  content  of  the 
soil.  Even  where  manure  is  available,  a  green-manuring 
crop  now  and  then  in  the  rotation  does  much  towards  sus- 
taining normal  production. 

The  effects  of  turning  under  green  plants  are  both  direct 
and  indirect — direct  as  to  the  influence  on  the  succeeding  crop, 
and  indirect  as  to  the  soil  so  treated.  In  the  first  place,  cer- 
tain ingredients  are  actually  added  to  the  soil  by  such  a 
procedure.  The  carbon,  oxygen,  and  hydrogen  of  plants  come 
Largely  from  the  air  and  water,  and  the  plowing-under  of  a 
crop,  therefore,  increases  the  store  of  such  constituents  in  the 
soil.  The  compounds  that  result  from  crop  decay  increase 
the  absorptive  power  of  the  soil,  and  promote  aeration,  drain- 
age, and  granulation — conditions  that  are  extremely  impor- 
tant in  successful  plant  growth.  If  the  crop  turned  under  is 
a  legume  and  the  nodule  organisms  are  active,  the  store  of  soil 
nitrogen  is  markedly  augmented,  a  point  of  extreme  impor- 
tance in  fertilizer  practice. 

Green-manures  may  function  also  as  cover-crops,  insofar  as 
they  take  up  the  extremely  soluble  plant  nutrients  and  pre- 
vent them  from  being  lost  in  the  drainage  water.  The  nitrates 
of  the  soil  are  of  particular  importance  in  this  regard  as  they 
are  very  soluble  and  are  absorbed  only  slightly  by  the  soil 
complexes.  Besides  this,  green-manures,  especially  those  with 
long  roots,  tend  to  carry  nutrients  upward  from  the  subsoil 
and  when  the  crop  is  turned  under  this  material  is  deposited 
within  the  root  zone.  Again,  the  added  organic  material  acts 
as  a  food  for  soil  organisms,  and  tends  to  stimulate  biological 
changes  to  a  marked  degree.  This  biological  action  is  espe- 
cially important  in  the  production  of  carbon  dioxide,  am- 
monia, nitrates,  and  organic  compounds  of  various  kinds, 
which  are  necessary  in  plant  nutrition. 


GREEN-MANURES  539 

310.  Gain  of  constituents  by  green-manuring. — In  an 
average  crop  of  green-manure,  from  five  to  ten  tons  of  mate- 
rial are  turned  under.  Of  this,  from  one  to  two  tons  are  dry- 
matter,  and  from  four  to  eight  tons  water.  Of  this  dry  matter, 
a  great  proportion  is  carbon,  hydrogen,  and  oxygen.  It  might 
seem  at  first  thought  that  such  an  addition  is  pure  gain  as 
far  as  carbon  and  carbonaceous  matter  are  concerned.  Such 
is  not  the  case.  Large  amounts  of  carbon  are  lost  continu- 
ously in  drainage,  to  say  nothing  of  that  removed  by  crops  or 
that  which  is  respired  by  the  soil  as  carbon  dioxide.  It  has 
already  been  shown,  from  results  obtained  with  the  Cornell 
lysimeters,  that  a  heavy  soil  will  yearly  lose  over  250  pounds 
of  carbon,  in  drainage  alone  (see  par.  220).  This  is  approxi- 
mately equivalent  to  a  2-ton  application  of  green-manure. 
Although  the  loss  of  carbonaceous  material  is  considerable, 
even  during  the  period  that  the  green-manuring  crop  is  being 
grown,  nevertheless  the  practice  offers  a  rapid  as  well  as  a 
natural  means  of  increasing  the  soil  organic  matter. 

The  mineral  parts  of  the  turned-under  crop  came  from  the 
soil  originally  and  they  are  merely  turned  back  to  it  again 
and  represent  ho  gain.  As  they  return,  however,  they  are  in 
intimate  union  with  organic  materials,  and  are  thus  readily 
available  as  the  decay  processes  go  on.  Indeed  they  are  prob- 
ably more  readily  available  than  they  previously  were,  when 
the  green-manuring  crop  acquired  them. 

The  amount  of  nitrogen  added  to  a  soil  if  the  green-manure 
is  a  legume  1  is  an  uncertain  quantity.  Much  depends  on  the 
virulence  of  the  organisms  occupying  the  nodules.    These  bac- 

1  Smith,  C.  D.,  and  Kobinson,  F.  W.,  Influence  of  Nodules  on  the  Boots 
upon  the  Composition  of  Soybean  and  Cowpea;  Mich.  Agr.  Exp.  Sta., 
Bui.  224,  1905. 

Hopkins,  C.  G.,  Alfalfa  on  Illinois  Soil;  111.  Agr.  Exp.  Sta.,  Bui.  76, 
1902. 

Hopkins,  C.  G.,  Nitrogen  Bacteria  and  Legumes;  111.  Agr.  Exp.  Sta., 
Bui.  94,  1904. 

Shutt,  F.  T.,  The  Nitrogen  Enrichment  of  Soils  through  the  Growth 
of  Legumes;  Canadian  Dept.  Agr.,  Kept.  Centr.  Exp.  Farms,  1905,  pp. 
127-132. 


540        NATURE  AND  PROPERTIES  OF  SOILS 

teria  are  in  turn  much  influenced  by  plant  and  soil  conditions, 
such  as  amount  of  organic  matter,  presence  of  nitrates,  acidity 
and  the  like.  Hopkins 1  estimates  that  about  one-third  of  the 
nitrogen  in  a  normal  innoculated  legume  comes  from  the  soil 
and  two-thirds  from  the  air.  He  also  considers  that  one-third 
of  the  nitrogen  exists  in  the  roots. 

Both  of  these  assumptions  are  questionable  and  at  best 
tentative.  The  amount  of  nitrogen  fixed  by  legume  organisms 
is  extremely  variable,  probably  more  so  than  that  assimilated 
by  the  azotobacter  and  allied  groups.  Again  the  percentage 
of  the  nitrogen  held  in  the  roots  of  legumes  is  by  no  means 
the  same  for  all  species.  The  amount  varies  within  the  species 
with  age,  degree  of  maturity  and,  season.  The  Delaware  in- 
vestigations 2  show  that  the  proportion  of  the  total  nitrogen 
of  the  plant  occurring  in  the  roots  may  be  as  low  as  6  per  cent, 
in  case  of  cowpeas  and  as  high  in  the  roots  of  alfalfa  as  42 
per  cent.  A  range  from  6  to  28  per  cent,  of  the  total  nitrogen 
of  crimson  clover  was  noted  in  the  roots  under  different  condi- 
tions. 

According  to  Hopkins,  the  nitrogen  found  in  the  tops  of 
legumes  will  be  a  rough  measure  of  the  nitrogen  fixed  by  the 
nodule  organisms.  When  the  crop  is  turned  under,  this  will 
represent  the  gain  to  the  soil.  If  the  preceding  assumption 
is  correct,  red  clover  turned  under  would  actually  add  about 
50  pounds  of  nitrogen  for  every  ton  of  air-dry  substance  util- 
ized, alfalfa  about  50,  cowpeas  43,  and  soybeans  53  pounds. 
These  figures,  even  though  they  may  be  far  from  correct,  at 
least  give  some  idea  of  the  possible  addition  of  nitrogen  by 
green-manuring  practices,  and  show  how  the  soil  may  be  en- 
riched by  such  management.  As  in  the  case  of  farm  manures, 
the  indirect  effects  of  such  a  procedure  on  the  physical  and 
bacteriological  properties  of  the  soil  may  over-ride  the  direct 

1  Hopkins,  C.  G.,  Soil  Fertility  and  Permanent  Agriculture,  p.  223; 
Boston,  1910. 

a  Penny,  C.  L.,  The  Growth  of  Crimson  Clover;  Del.  Agr.  Exp.  Sta., 
Bui.  67,  1905.  8 


GREEN-MANURES  541 

influences,  lessening  the  advantage  that  legumes  as  green- 
manures  are  supposed  to  have  over  non-legumes,  due  to  their 
ability  to  use  atmospheric  nitrogen. 

311.  Green-manures  as  cover-crops. — When  green-ma- 
nures are  seeded  in  the  late  summer  or  early  fall,  they  func- 
tion as  cover-crops  and  may  have  rather  important  influences 
aside  from  their  effects  when  turned  under.  Their  greatest 
influence  seems  to  be  on  the  nitrate  content  of  the  soil.  Nitri- 
fication is  usually  checked,1  a  disappearance  of  nitrates  gen- 
erally following.  This  reduction  in  the  amount  of  nitrates 
probably  occurs  because  of  a  retardation  of  nitrification  ac- 
companied by  a  stimulation  of  biological  utilization  of  the 
nitrates.  Such  an  effect  is  important  in  conserving  the  soil 
nitrogen  and  is  of  particular  value  in  orchards,2  as  it  hastens 
the  maturity  of  the  new  growth.  At  Cornell  University, 
green-manures  were  seeded  in  July  and  plowed  under  in  the 
following  spring.  Nitrate  determinations  were  made  on  the 
soil  in  July  and  in  October.  The  figures  are  five-year  aver- 
ages.    (See  Table  CXXIII,  page  542.) 

312.  The  decay  of  green-manure. — When  a  green-crop 
is  turned  under,  the  process  of  its  decay  is  the  same  as  that 
of  any  plant  tissue  that  becomes  a  part  of  the  soil  body.  The 
organisms  that  are  active  are  those  common  to  the  soil,  to- 
gether with  such  bacteria  as  are  carried  into  the  soil  on  the 
turned-under  crop.  The  decomposition  that  results  is  prob- 
ably both  aerobic  and  anaerobic  in  nature,  carbon  dioxide  be- 
ing given  off  continuously.  When  proper  decay  has  occurred, 
end  products  should  result  which  can  be  utilized  as  nutrients. 

1  Wright,  E.  C,  The  Influence  of  Certain  Organic  Materials  upon  the 
Transformation  of  Soil  Nitrogen;  Amer.  Soc.  Agron.,  Vol.  7,  pp.  193- 
208,  1915. 

Martin,  T.  L.,  The  Decomposition  of  Green  Manures  at  Different  Stages 
of  Growth;  Thesis  for  degree  of  Doctor  of  Philosophy,  Cornell  University, 
1919. 

2  Lyon,  T.  L.,  The  Formation  of  Nitrates  in  Soil  Under  Grass; 
Proc.  West.  N.  Y.  Hort.  Soc,  pp.  82-87,  Jan.,  1915. 

Lyon,  T.  L.,  Relation  of  Certain  Cover  Crops  to  the  Formation  of 
Nitrates  in  Soil;  Proc  West.  N.  Y.  Hort.  Soc,  pp.  32-34,  Jan.,  1917. 


542        NATURE  AND  PROPERTIES  OF  SOILS 


Table  CXXIII 

EFFECT   OF  VARIOUS   CROPS   ON    THE   NITRATE   NITROGEN    OF   THE 
SOIL  DURING  OCTOBER,  1916-1920.1 


Green-Manuring  Crop 


Rye 

Oats 

Vetch 

Peas 

Rye  and  vetch 
Rye  and  peas. 
Sod 


Nitrates  in  the 

Soil  in  October. 

Eye  Taken  as 

100 


100 
73 
73 
83 
74 
75 
6 


Percentage  Se- 
duction of 
Nitrates  in 
October  Compared 
with  July 


37 
44 
57 
10 

58 

58 

0 


The  intermediate  compounds  that  are  formed  should  yield 
an  organic  matter  carrying  a  black  pigment,  should  readily 
split  up  into  simple  compounds,  and  should  be  in  general 
beneficial,  both  directly  and  indirectly,  to  plant  growth. 
Plenty  of  moisture  is  essential  when  green-manures  are  de- 
caying, not  only  to  hasten  the  transformation  itself  but  that 
the  normal  soil  processes  may  not  be  interrupted  by  a  lack 
of  water.  The  caution  with  which  green-manures  must  be 
utilized  in  semi-arid  regions  arises  because  of  the  drying  influ- 
ences of  rapid  decay  and  the  danger  of  filling  the  soil  with 
undecomposed  plant  residues.  Even  in  humid  regions,  green- 
manures  may  be  detrimental  if  dry  weather  sets  in  before  a 
major  portion  of  the  decay  processes  is  completed. 

As  plant  tissue  decays  in  the  soil,  there  seem  to  be  two 
general  groups  of  forces  at  work  which  produce  three  distinct 
stages  of  organic  destruction.2    In  the  first  stage,  humus  pro- 

1  Unpublished  data.    Dept.  Soils.     Cornell  University. 

2  Martin,  T.  L.,  The  Decomposition  of  Green-Manures  at  Different 
Stages  of  Growth;  Thesis  for  degree  of  Doctor  of  Philosophy,  Cornell 
University,  1919. 


GREEN-MANURES  543 

duction  is  dominant  and  the  amount  of  the  humous  materials 
increases.  In  the  second  stage,  humus  production  and  humus 
destruction  are  more  or  less  balanced,  while  in  the  third  stage 
humus  destruction  is  in  the  ascendant.  The  amount  of  humus 
is  on  the  decrease  in  the  latter  stage.  The  length  of  these 
stages  will  vary  with  the  season,  with  soil  conditions,1  and 
with  the  character  of  the  crop  turned  under.  Obviously,  the 
influence  of  decomposing  green-manure  on  the  chemical  and 
biological  activities  of  the  soil  will  vary  as  the  decay  cycle 
progresses.  In  general,  over  one-half  of  the  organic  matter 
of  the  average  green-manure  disappears  during  the  first  nine 
months  after  application. 

313.  Influence  of  decaying  green-manure. — In  the  first 
stage  of  decay,  which  should  be  a  rapid  one,  many  complex 
compounds  are  generated  along  with  carbon  dioxide  and  other 
simple  products.  The  complex  materials,  which  result  partly 
from  protein  decomposition  and  partly  from  the  breaking 
down  of  easily  attacked  carbohydrates,  may  be  harmful  to 
ordinary  crops.  Germinating  seeds  and  young  plants  are 
especially  susceptible,  and  detrimental  influences  are  some- 
times noticed  immediately  after  the  turning  under  of  a  green- 
manure.  Fred 2  found  that  the  germination  of  oily  seeds, 
such  as  cotton  and  soybean,  was  much  reduced.  Starchy 
seeds,  such  as  maize,  oats,  and  wheat,  were  little  affected.  The 
germination  of  flax,  hemp,  mustard,  and  clover  was  some- 
what reduced.  An  actual  contact  of  the  seed  with  the  de- 
caying material  was  usually  necessary  for  serious  damage. 
The  detrimental  influence  always  occurred  during  the  first 
two  or  three  weeks  after  the  green-crop  was  turned  under. 
Obviously  the  more  succulent  the  crop,  the  shorter  will  this 
period  be. 

1Eussell,  E.  J.,  and  Appleyard,  A.,  The  Influence  of  Soil  Conditions 
on  the  Decomposition  of  Organic  Matter  in  the  Soil;  Jour.  Agr.  Sci., 
Vol.  VIII,  Part  3,  pp.  385-417,  1917. 

3  Fred,  E.  B.,  Relation  of  Green  Manure  to  the  Failure  of  Certain 
Seedlings;  Jour.  Agr.  Res.,  Vol.  V,  No.  25,  pp.  1161-1176,  Mar.,  1916. 


544        NATURE  AND  PROPERTIES  OF  SOILS 

Not  only  do  the  products  of  the  first  stage  of  decay  influ- 
ence the  crop  growing  on  the  soil,  but  they  affect  the  biological 
activities  as  well.1  Nitrification  in  particular  seems  to  be  in- 
fluenced, as  nitrates  do  not  begin  to  appear  until  the  process 
of  humification  is  well  advanced.  Nitrification,  however,  is 
probably  not  entirely  suppressed  as  it  is  possible  for  soil  or- 
ganisms to  use  up  the  nitrates  as  rapidly  as  they  are  formed. 


TIME  AFTER   APPLICATION 

Fig.  64. — Diagram  illustrating  the  three  stages  in  the  decay  of  a 
green-manure.  I,  humus  production  dominant;  II,  a  balance  be- 
tween humus  production  and  destruction;  III,  humus  destruction 
dominant.  A  depression  in  nitrate  accumulation  generally  occurs 
in  stage  I  followed  by  an  increase.     (After  Martin.) 


As  the  humus  destruction  gradually  dominates  over  humus 
production,  the  end  products  of  the  decay  become  prominent. 
The  complex  proteid  decomposition  is  practically  completed 
and  cellulose  destruction  is  slowly  progressing.  Of  the  sim- 
ple nutritive  products,  the  nitrates  are  of  particular  impor- 
tance.    In  fact,  they  have  been  chosen  by  a  number  of  in- 


1  Briscoe,  C.  F.,  and  Harned,  H.  H.,  Bacterial  Effects  of  Green 
Manures;  Miss.  Agr.  Exp.  Sta.,  Bui.  168,  Jan.  1915. 

Hutchinson,  H.  B.,  The  Influence  of  Plant  Residues  on  Nitrification 
and  on  Losses  of  Nitrates  in  Soil;  Jour.  Agr.  Sci.,  Vol.  IX,  Part  1, 
pp.  92-111,  Aug.  1918. 


GREEN-MANURES  545 

vestigators1  as  a  measure  of  humification,  since  a  favorable 
environment  for  nitrification  probably  does  not  occur  until 
the  more  rapid  decomposition  processes  are  completed.  In 
general,  the  more  rapid  the  decay  of  the  green-manure,  the 
sooner  will  nitrification  be  active  again. 

Besides  affecting  the  bacterial  activity  of  the  soil,  the  de- 
caying green-crop  influences  the  solubility  of  the  soil  min- 
erals. Jensen 2  found  that  the  addition  of  3  per  cent,  of 
green-manure  raised  the  solubility  of  lime  and  phosphoric 
acid  30  to  100  per  cent.  This  was  over  and  above  the  mineral 
constituents  which  came  directly  from  the  decomposing  green- 
crop.    Magnesium  and  iron  were  also  markedly  influenced. 

314.  Crops  suitable  for  green-manures. — An  ideal  green- 
manuring  crop  should  possess  three  characteristics:  rapid 
growth,  abundant  and  succulent  tops,  and  the  ability  to  grow 
well  on  poor  soils.  The  more  rapid  the  growth,  the  greater 
the  chance  of  economically  using  such  a  crop  as  a  means  of 
soil  improvement.  The  higher  the  moisture  content  of  the 
crop,  the  more  rapid  the  decay  and  the  more  quickly  are  bene- 
fits obtained.  As  the  need  of  organic  matter  is  especially 
urgent  on  poor  land,  a  hardy  crop  has  great  advantages. 

The  crops  that  may  be  utilized  as  green-manures  are  usually 

1  Hutchinson,  C.  M.,  and  Milligan,  S.,  Green-Manuring  Experiments, 
1912  and  1913.    India  Agr.  Ees.  Inst.  Bui.  40,  Pusa,  India,  1914. 

Maynard,  L.  A.,  The  Decomposition  of  Sweet  Clover  as  a  Green- 
Manure  under  Greenhouse  Conditions;  Cornell  Agr.  Exp.  Sta.,  Bui.  No. 
394,  1917. 

Martin,  T.  L.,  The  Decomposition  of  Green-Manures  at  Different 
Stages  of  Growth;  Thesis  for  degree  of  Doctor  of  Philosophy,  Cornell 
University,  1919. 

3  Jensen,  C.  A.,  Effect  of  Decomposing  Organic  Matter  on  the  Solu- 
bility of  Certain  Inorganic  Constituents  of  the  Soil;  Jour.  Agr.  Ees., 
Vol.  IX,  No.  8,  pp.  253-268,  May  1917. 

See  also,  Snyder,  H.,  Humus  as  a  Factor  in  Soil  Fertility;  Minn.  Agr. 
Exp.  Sta.,  Bui.  41,  1895;  and  Production  of  Humus  from  Manures; 
Minn.  Agr.  Exp.  Sta.,  Bui.  53,  1897. 

Hopkins,  C.  G.,  and  Aumer,  J.  P.,  Potassium  from  the  Soil;  111.  Agr. 
Exp.  Sta.,  Bui.  182,  1915. 

Hopkins,  C.  G.,  and  Whiting,  A.  L.,  Soil  Bacteriology  and  Phosphates; 
111.  Agr.  Exp.  Sta.,  Bui.  190,  1916. 


546 


NATURE  AND  PROPERTIES  OF  SOILS 


grouped  under  two  heads,  legumes  and  non-legumes.     Some 
of  the  common  green-manures  are  as  follows: 


LEGUMES 

NON-LEGUMES 

Annual 

Biennial 

Cowpea 

Red  clover 

Rye 

Soybean 

White  clover 

Oats 

Peanut 

Alsike  clover 

Mustard 

Vetch 

Alfalfa 

Mangels 

Canada  field 

pea 

Sweet  clover 

Rape 

Velvet  bean 

Buckwheat 

Crimson  clover 

Hairy  vetch 

When  other  conditions  are  equal,  it  is  of  course  always  bet- 
ter to  choose  a  leguminous  green-manure  in  preference  to  a 
non-leguminous  one,  because  of  the  nitrogen  that  may  be 
added  to  the  soil.  However,  it  is  so  often  difficult  to  obtain 
a  catch  of  some  of  the  legumes  that  it  is  poor  management  to 
turn  the  stand  under  until  after  a  number  of  years.  Again, 
the  seed  of  many  legumes  is  very  expensive,  almost  prohibit- 
ing their  use  as  green-manures.  Among  the  legumes  most 
commonly  grown  as  green-manures,  cowpeas,  soybeans,  and 
peanuts  may  be  named.  Many  of  the  other  legumes  do  not  so 
fit  into  the  common  rotations  as  to  be  turned  under  conven- 
iently as  a  green-manure. 

For  the  reasons  already  cited,  the  non-legumes  have,  in 
many  cases,  proved  the  more  popular  and  economic  as  green- 
manures.  Rye  and  oats  are  much  used  because  of  their  rapid, 
abundant,  and  succulent  growth  and  because  they  may  be 
accommodated  to  almost  any  rotation.  They  are  hardy  and 
will  start  in  almost  any  kind  of  a  seed-bed.  They  are  thus 
extremely  valuable  on  poor  soils.  '  Often  the  value  of  such  a 
green-manure  as  oats  is  greatly  increased  by  sowing  peas  with 
it.  The  advantages  of  a  legume  and  a  non-legume  are  thus 
combined. 

It  has  already  been  shown  that  the  nitrate  production  in  a 


GREEN-MANURES 


547 


soil  may  be  used  as  a  rough  measure  of  the  rate  of  decay  of 
green-manures.  Admitting  such  a  criterion,  certain  data  from 
Cornell  University  become  particularly  interesting.  In  a 
five-year  continuous  test,  green-manuring  crops  were  seeded 
in  July  and  plowed  under  in  the  early  part  of  the  succeeding 
May.  The  nitrate  content  of  the  soil  was  determined  at  a 
number  of  times  during  the  spring,  summer,  and  fall.  A  de- 
crease in  nitrates  always  occurred  in  the  autumn,  while  an 
increase  began  soon  after  the  crops  were  turned  under  in  the 
spring.  In  the  following  table  the  rye  crop  is  taken  as  100  in 
both  October  and  July: 


i/£i 


Table  CXXIV 

RELATIVE  INFLUENCE  OF  GREEN-MANURES  ON  THE 
ACCUMULATION  OF  SOIL  NITRATES.1 


Green-Manure 


Rye 

Oats 

Veteh 

Peas 

Rye  and  vetch 
Rye  and  peas. 


Nitrates  in  July, 

Nitrates  in  Oct., 

Soil  Fallow  Since 

Soil  Under  Crop 

May  1. 

Since  July. 

Rye  Taken  as  100 

Eye  Taken  as  100 

100 

100 

78 

73 

120 

73 

99 

83 

136 

74 

102 

75 

It  is  immediately  apparent  that  the  succulent  rye  and  vetch 
that  survive  the  winter  give  better  results,  as  far  as  nitrate 
production  is  concerned,  than  the  dry  and  dead  oats  and  peas. 
This  shows  clearly  the  value  of  succulence  and  the  necessity 
of  turning  under  a  crop  partially  matured.2  The  advantage 
of  the  legumes  over  the  non-legumes  is  not  hard  to  explain. 


1  Unpublished  data.     Dept.  Soils,  Cornell  University. 

3  Martin,  T.  L.,  The  Decomposition  of  Green  Manures  at  Different 
Stages  of  Growth;  Thesis  for  the  Degree  of  Doctor  of  Philosophy, 
Cornell  University,  1919. 


548        NATURE  AND  PROPERTIES  OF  SOILS 

The  combination  of  rye  and  vetch,  both  of  course  in  a  succu- 
lent condition,  seems  especially  efficacious.  Sod  as  a  green- 
manure  always  appears  more  or  less  at  a  disadvantage. 

315.  The  use  of  green-manures. — The  indiscriminate 
use  of  green-manures  is  of  course  never  to  be  advised,  as  the 
soil  may  be  injured  thereby  and  the  normal  rotation  much 
interfered  with.  When  soils  are  poor  in  nitrogen  and  organic 
matter,  they  are  very  often  in  poor  tilth.  This  is  true  whether 
the  texture  of  the  soil  be  fine  or  coarse.  The  turning-under 
of  green-crops  must  be  judicious,  however,  in  order  that  the 
soil  may  not  be  clogged  with  undecayed  matter.  Once  or  twice 
in  a  rotation  is  usually  enough  for  such  treatments.  Proper 
drainage  must  always  be  provided.  In  regions  where  the  rain- 
fall is  scanty,  great  caution  must  be  observed  in  the  handling 
of  green-manures.  The  available  moisture  that  should  go  to 
the  succeeding  crop  may  be  used  in  the  process  of  decay,  and 
the  soil  left  light  and  open,  due  to  an  excess  of  undecomposed 
plant  tissue.  In  western  United  States,  it  is  still  a  question 
whether  green-manures  have  any  advantage  over  summer 
fallowing. 

It  is  generally  best  to  turn  under  green-crops  when  their 
succulence  is  near  the  maximum  and  yet  at  a  time  when 
abundant  tops  have  been  produced.  This  occurs  at  about  the 
half  mature  stage.  A  large  quantity  of  water  is  carried  into 
the  soil  when  the  crop  is  at  this  stage,  and  the  draft  on  the 
original  soil-moisture  is  less.  Again,  the  succulence  encour- 
ages a  rapid  and  more  or  less  complete  decay,  with  the  maxi- 
mum production  of  humus  and  other  products.  The  plowing 
should  be  done,  if  possible,  at  a  season  when  a  plentiful  supply 
of  rain  occurs.  The  effectiveness  of  the  manuring  is  thereby 
much  enhanced.  At  Cornell  University  various  green-manures 
were  seeded  in  the  summer  and  plowed  under  that  fall  or  the 
next  spring.  The  experiment  was  continuous  for  three  years, 
the  nitrates  being  determined  in  the  soil  each  year  in  April 
and  in  June.    The  results  are  as  given  on  the  next  page. 


GREEN-MANURES 


549 


Table  CXXV 

INFLUENCE  OF  THE  TIME  OF  TURNING-UNDER  OF  GREEN-MANURES 
ON  THE  NITRATE  ACCUMULATION  IN  THE  SOIL.1 


Parts  Per  Million  of  Nitrates 

Crop 

In  April  Just 

Before  the 
Spring  Plowing 

In  June,  Soil 

Fallowed  Since 

Plowing 

Rye,  fall  plowed 

Rye,  spring  plowed 

Oats,  fall  plowed 

58 
53 

61 
36 

79 
41 

66 
43 

57 
67 

42 

Oats,  spring  plowed 

Vetch,  fall  plowed 

50 
45 

Vetch,  spring  plowed 

Average,  fall  plowed 

Average,  spring  plowed 

67 

48 
61 

It  is  apparent  that  the  decay  of  the  green-manuring  crop 
is  hastened  by  fall  plowing,  as  the  nitrates  in  every  case  are 
higher  in  April  on  land  so  handled.  In  June,  however,  the 
nitrate  accumulation  has  passed  its  highest  point  in  the  fall- 
plowed  soil,  leaving  the  spring-plowed  plats,  where  the  decay 
was  initiated  later,  in  the  ascendancy.  The  table  also  shows 
the  advantage  that  a  legume  has  over  a  non-legume  in  causing 
nitrate  accumulation.  Oats  fall-plowed  appear  about  on  an 
equality  with  rye.  Spring  plowing,  since  the  oats  are  then 
dry  and  dead,  gives  the  rye  a  marked  advantage.  All  of  the 
points  above  noted  have  a  very  practical  field  application. 

In  turning  under  green-manures,  the  furrow  slice  should 
not  be  thrown  over  flat,  since  the  green-crop  is  then  deposited 
as  a  continuous  layer  between  the  surface  soil  and  the  sub- 
soil.    Capillary  movement  is  thus  impeded  until  a  more  or 

1  Unpublished  data.    Dept.  Soils,  Cornell  University. 


550        NATURE  AND  PROPERTIES  OF  SOILS 

less  complete  delay  has  occurred,  and  the  succeeding  crop 
may  suffer  from  lack  of  moisture.  The  furrow  ordinarily 
should  be  turned  only  partly  over,  and  thrown  against  and 
on  its  neighbor.  The  green-manure  is  then  distributed  evenly 
from  the  surface  downward  to  the  bottom  of  the  furrow. 
When  decomposition  occurs,  the  resulting  materials  are  evenly 
mixed  with  the  whole  furrow  slice.  Moreover,  this  method  of 
plowing  does  not  interfere  with  the  capillary  movements  of 
water,  and  in  actual  practice  is  a  great  aid  in  drainage  and 
aeration. 

316.  Green-manure  and  lime. — The  decay  of  organic 
matter  in  the  soil  is  always  accompanied  by  the  production  of 
organic  acids  of  various  kinds.  The  greater  the  succulence 
of  the  material,  the  more  rapid  is  the  accumulation  of  such 
products.  In  spite  of  this,  however,  the  effect  of  a  green- 
manure  is  to  decrease  the  acidity  rather  than  increase  *  it  and 
later  greatly  to  stimulate  nitrification  even  if  the  soil  origi- 
nally was  quite  acid.  The  decrease  in  lime  requirement  may 
be  due  to  the  liberation  of  mineral  constituents  from  the  de- 
caying organic  matter  and  to  the  effect  of  the  decomposition 
on  the  inorganic  constituents  of  the  soil. 

The  ultimate  influence  of  green-manure  on  acidity  is  some- 
what in  doubt.  The  bulk  of  the  evidence  available  seems  to 
indicate  that  decaying  organic  matter,  if  it  has  any  effect,  ulti- 
mately tends  to  decrease  rather  than  increase  the  lime  re- 
quirement of  the  soil.2  Nevertheless,  plenty  of  active  calcium 
should  be  in  the  soil,  since  it  promotes  the  decay  of  the  plant 
tissue  added  and  seems  to  control  to  a  certain  extent  the  pres- 
ence of  toxic  materials.  Lime  may  be  added  to  the  green- 
manure  seeding  and  be  turned  under  with  that  crop.     The 

1  White,  J.  W.,  Soil  Acidity  as  Influenced  by  Green  Manures;  Jour. 
Agr.  Res.,  Vol.  XIII,  No.  3,  pp.  171-197,  April,  1918. 

2  Hill,  H.  H.,  A  Comparison  of  Methods  for  Determining  Soil  Acidity 
and  a  Study  of  the  Effects  of  Green  Manures  on  Soil  Acidity;  Va. 
Poly.  Inst.,  Tech.  Bui.  19,  April  1919. 

Ames,  J.  W.,  and  Schollenberger,  C.  J.,  Liming  and  Lime  Require- 
ment of  Soils;  Ohio  Agr.  Exp.  Sta.,  Bui.  306,  pp.  381-383,  Dec.  1916." 


GREEN-MANURES 


551 


amendment  would  thus  be  in  very  close  contact  with  the  de- 
caying vegetable  tissue.  Ordinarily,  however,  the  application 
of  lime  at  some  point  in  the  rotation  is  sufficient. 

Lime,  besides  its  capacity  to  alleviate  toxic  residues,  tends 
to  hasten  organic  decay.1  This  is  a  very  important  function 
as  the  first  stage  of  decomposition,  during  which  soil  and  plant 
activities  may  under  certain  conditions  be  detrimentally  af- 
fected, is  markedly  shortened.  Such  a  promotion  is  indicated 
in  a  green-manuring  experiment  at  Cornell  University.  The 
green-manures  were  seeded  in  the  fall  under  two  treatments, 
limed  and  unlimed.  The  parts  per  million  of  nitrates  in  the 
soil  are  given  for  two  dates  on  the  year  succeeding,  the  green- 
manures  having  been  plowed  under  either  in  the  fall  or  early 
spring.    The  data  are  averages  of  three  years. 

Table  CXXVI 

INFLUENCE  OF  LIME  ON  THE  NITRATE  ACCUMULATION  IN  A  SOIL 
RECEIVING  VARIOUS  GREEN- MANURES.2 


Crop  and  Treatment 

Parts  Per  Million  of  Nitrates 

April 

June 

Rye,  no  lime 

66 
45 

53 
45 

77 
43 

65 
44 

53 

Rye,  limed 

71 

Oats,  no  lime 

43 

Oats,  limed 

50 

Vetch,  no  lime 

52 

Vetch,  limed 

63 

Average,  no  lime 

49 

Average,  limed 

61 

1  Lemmermann,  O.,  et  al.,  Untersuchung  iiber  die  zerzetzung  der  Kohlen- 
stoff  YerMndungen  Verscheidener  Organischen  Substanzen  im  Boden 
Spezielle  unter  dem  einfluss  der  Kalk;  Landw.  Jahrb.,  Bd.  41,  S.  216- 
257,  1911. 

2  Unpublished  data.     Dept.  Soils,  Cornell  University. 


552        NATURE  AND  PROPERTIES  OF  SOILS 

The  effect  of  lime  on  nitrification  is  very  noticeable  in  June. 
In  April  the  no-lime  plats  are  higher  in  accumulated  nitrates, 
due  to  the  lesser  growth  of  the  green-manuring  crop. 

317.  Practical  utilization  of  green-manures.— Green- 
manures  seem  to  have  their  greatest  value  where  a  permanent 
instead  of  a  rotation  pasture  is  used,  where  a  long  cycle  rota- 
tion of  grain  is  practiced,  or  where  little  or  no  manure  is 
available.  The  experimental  data  bearing  on  the  use  of  green- 
manures  seems  to  indicate  that  such  a  practice  is  productive 
of  larger  crop  yields.  The  following  data  from  Nappan,  Nova 
Scotia,  is  from  one  of  the  more  reliable  and  conclusive  experi- 
ments. A  catch-crop  of  clover  in  the  grain  was  turned  under 
for  grain  the  following  year.  The  figures  are  for  1905,  the 
third  year  of  the  test. 

Table  CXXVII 

YIELD   OF  WHEAT,   OATS  AND  BARLEY   IN   BUSHELS  TO  THE  ACRE 

ON  THE  NAPPAN  FARM  IN  1905  ON  PLATS  CROPPED 

CONTINUOUSLY  TO  GRAIN.1 


Treatment 

Wheat 

Oats 

Barley 

No  green-manure 

Clover  catch-crop 

34.3 
40.0 

41.2 
55.3 

32.7 
37.9 

The  use  of  a  green-manure  is  often  determined  by  the  char- 
acter of  the  rotation.  Very  often  it  is  somewhat  of  a  problem 
as  to  when,  in  an  ordinary  rotation,  a  green-manure  may  be 
introduced  so  that  it  may  fit  in  well  with  the  crops.  In  a 
rotation  of  maize  or  potatoes,  oats,  wheat,  and  two  years  of 
hay,  a  green-manure  might  be  introduced  after  the  corn  or 
potatoes.  This  would  not  be  a  very  good  practice,  however,  as 
a  cultivated  crop  usually  should  follow  a  green-manure  in 
order  to  facilitate  decomposition  and  decay.  In  such  a  rota- 
tion, the  plowing-under  of  the  hay  stubble  is  really  a  form 


1  Ottawa  Exp.  Farms  Kept,,  1905,  p.  284. 


GREEN-MANURES  553 

of  green-manuring,  there  being  a  considerable  accumulation 
of  stubble  and  aftermath  on  the  soil.  When  a  rotation  of 
this  kind  is  used,  it  is  better  either  to  supply  organic  matter 
in  other  ways,  or  to  alter  or  break  the  rotation  in  such  a  man- 
ner as  to  admit  of  a  more  advantageous  use  of  green-crops. 

Where  trucking  crops  are  raised  and  no  very  definite  rota- 
tice  is  adhered  to,  green-manuring  is  easier.  It  is  especially 
facilitated  when  cover-crops  are  grown,  as  in  orchards.  Soil- 
ing operations  also  favor  the  easy  and  profitable  use  of  green- 
manures.  In  general,  it  may  be  said  that  the  organic  matter 
obtained  from  such  a  source  should  be  supplemented  by  farm- 
yard manures  where  possible.  A  better  balanced  and  richer 
soil  organic  matter  is  more  likely  to  result. 


CHAPTER  XXVI 
TEE  MAINTENANCE  OF  SOIL  FERTILITY l 

The  maintenance  of  a  profitable  and  continuous  soil  pro- 
ductivity is  an  intricate  problem,  since  many  variable  factors 
are  involved.  Weather  conditions,  moisture  relations,  soil  or- 
ganic matter  and  tilth,  plant  diseases,  soil  reaction,  and  avail- 
able nutrients  are  only  a  few  of  the  influences  that  function 
continuously  throughout  the  growing  season.  No  scheme  of 
soil  management  and  crop  production  is  perfect,  even  though 
it  is  fairly  profitable.  Except  in  special  cases,  every  system  is 
open  to  improvement  and  modification  as  soil  and  plant 
knowledge  increases. 

The  sources  of  knowledge  regarding  the  profitable  growing 
of  plants  are  numerous.  Much  data  have  arisen  from  expe- 
rience and  observation,  much  are  empirical,  while  some  are 
confessedly  conjectural.  In  spite  of  the  large  amount  of 
scientific  information  available  regarding  the  soil  and  its 
plant  relationships,  practical  experience  has  contributed  more 
towards  a  profitable  and  continuous  soil  productivity.  Soil 
survey  classification  and  mapping  have  contributed  some- 
thing. Field  tests,  both  practical  and  technical,  have  added 
to  such  information,  while  laboratory  and  greenhouse  experi- 
ments, although  often  arbitrary  and  artificial,  are  by  no 
means  unimportant.  These  latter  contributions,  however, 
always  need  practical  confirmation  under  typical  field  con- 
ditions over  a  period  of  years. 

318.  Loss  of  plant  nutrients  from  the  soil. — A  consid- 
eration of  the  principles  governing  the  rational  management 

1  Fertility  is  here  used  in  the  sense  of  continuous  productivity. 

554 


THE  MAINTENANCE  OF  SOIL  FERTILITY     555 

of  a  soil  is  obviously  impossible  unless  some  knowledge  is  at 
hand  regarding  the  losses  and  additions  which  a  soil  sustains 
in  the  course  of  a  definite  ^rotation.  Fortunately,  some  fairly 
reliable  data  have  already  been  presented  regarding  the  re- 
moval of  soil  constituents  under  controlled  conditions.  The 
Cornell  lysimeter  tanks,  bearing  a  rotation  of  maize,  oats, 
wheat,  and  two  years  of  hay,  offer  very  satisfactory  informa- 
tion (paragraphs  95  and  163).  The  losses  covering  a  ten-year 
period  are  expressed  in  pounds  to  the  acre  a  year.  The  soil  is 
a  Dunkirk  silty  clay  loam. 

While  such  figures  are  probably  open  to  considerable  error 
and  obviously  would  not  apply  with  any  degree  of  accuracy 
to  a  light  soil,  they  indicate  in  a  general  way  the  magnitude 
and  order  of  the  losses  that  may  be  expected  from  such  a  soil 
under  the  conditions  specified. 


Table  CXXVIII 

LOSSES  FROM  A  DUNKIRK   SILTY   CLAY  LOAM   SOIL  EXPRESSED  IN 

POUNDS  TO  THE  ACRE  A  YEAR  OVER  A  TEN- YEAR  PERIOD. 

ROTATION:      MAIZE,   OATS,  WHEAT  AND  TWO  YEARS 

HAY.       CORNELL   LYSIMETER    TANKS. 


Source  of  Loss 

N 

p2o6 

K20 

CaO 

SO, 

Drainage  (par.  163) .... 

Cropping  (par.  163) 

Atmosphere   (pars. 

220  and  233)1 

7.3 
70.5 

trace 
43.5 

68.7 
105.4 

345.9 
24.3 

108.5 
41.0 

Total 

77.8 

43.5 

174.1 

370.2 

149.5 

The  organic  carbon  in  this  soil  over  the  ten-year  period  was 
reduced  at  the  rate  of  approximately  1  per  cent,  a  year.2  This 
is  equivalent  to  a  reduction  in  organic  matter  of  about  1200 

xThe  largest  loss  of  carbon  is  probably  to  the  atmosphere  as  carbon 
dioxide.     The  other  avenue  of  loss  is  in  the  drainage  water. 

aLipman  and  Blair  report  a  reduction  of  organic  carbon  of  .74  per 
cent,  a  year  over  a  period  of  ten  years  on  Sassafrass  loam  in  New 


556        NATURE  AND  PROPERTIES  OF  SOILS 

pounds  each  year  to  the  acre-four  feet.  It  is  evident,  there- 
fore, that  the  losses  sustained  by  the  average  soil  fall  most 
heavily  on  the  organic  constituents,  a  condition  often  ignored 
in  practical  soil  management.  The  removal  of  calcium  oxide  is 
also  very  large,  being  equivalent  to  a  loss  of  661  pounds  of 
calcium  carbonate  an  acre  a  year.  Although  losses  of  sulfur 
trioxide  and  phosphoric  acid  are  smaller  than  that  of  the 
potash,  they  are  far  more  important,  since  there  is  very  com- 
monly one  hundred  times  more  potash  in  a  soil  than  of  the 
other  two  constituents  combined.  The  magnitude  of  the  loss 
of  a  soil  constituent  is  never  a  safe  measure  of  its  importance. 
The  removal  of  nitrogen  is  equivalent  to  over  500  pounds  of 
commercial  sodium  nitrate  and  consequently  is  also  a  loss  of  no 
small  consideration. 

319.  Additions  of  nutrients  to  the  soil. — The  figures 
presented  above  are  based  on  reliable  experimental  data.  Un- 
fortunately the  information  regarding  the  additions  which 
normally  occur  to  a  soil  under  any  particular  rotation  are  by 
no  means  so  exact.  Certain  assumptions  and  estimates,  often 
of  questionable  validity,  must  be  admitted  in  order  that  a 
complete  survey  may  be  possible.  Table  CXXIX  sets  forth 
the  additions  which  the  Dunkirk  clay  loam  of  the  Cornell  lysi- 
meters  may  reasonably  be  expected  to  receive  each  year  when 
cropped  to  a  five-year  rotation  of  maize,  oats,  wheat,  and  two 
years  hay.  The  data  are  expressed  in  pounds  to  the  acre  a 
year.     (See  Table  CXXIX,  page  557.) 

The  additions  listed  above  are  not  the  only  avenues  open 
for  important  acquisitions.  The  crops  removed  may  be  fed  to 
animals  and  the  manure  returned  to  the  land.  Moreover,  the 
utilization  of  a  green-manure  is  also  possible.  Below  will  be 
found  the  additions  that  may  reasonably  be  expected  from  the 

Jersey.  The  rotation  was  maize,  oats,  wheat,  and  two  years  hay.  No 
lime  was  added. 

Lipman,  J.  G.,  and  Blair,  A.  W.,  The  Lime  Factor  in  Permanent 
Soil  Improvement.  I.  Rotations  Without  Legumes;  Soil  Sci.,  Vol. 
IX,  No.  2,  p.  87,  1921. 


THE  MAINTENANCE  OF  SOIL  FERTILITY     557 
Table  CXXIX 

ESTIMATED   ADDITIONS   THAT    MIGHT   OCCUR  TO  A   SOIL   UNDER   A 

ROTATION   OF    MAIZE,    OATS,    WHEAT,   AND    TWO   YEARS    HAY. 

EXPRESSED  IN  POUNDS   TO   THE  ACRE  A  YEAR. 


Source  of  Addition 

N 

PA 

K20 

CaO 

so3 

Rain-water  (pars.  236  and  264) 
Free  fixation  by  soil  organisms 

(par.  238) 

Crop-roots  and  residues  1 

12.5 
25.0 

37.5 







65.0 

Total 





65.0 

use  of  farm  manure  and  a  green-manure  on  the  soil  in  question. 
The  green-manure  is  leguminous  and  is  applied  once  during 
the  five-year  rotation. 

Table  CXXX 

FURTHER    ADDITIONS    THAT    MIGHT    BE    MADE    TO    THE   FIVE-YEAR 

ROTATION  ON  DUNKIRK  SILTY  CLAY  LOAM,  EXPRESSED  IN 

POUNDS    TO   THE   ACRE   A   YEAR. 


Additions 

N 

PA 

K20 

CaO 

so3 

Organic 
Matter 

Farm  manure  2 

Leguminous  green- 
manure  3 

21.1 
20.0 

21.7 

31.6 

7.3 

12.4 

1000 
600 

Total 

41.1 

21.7 

31.6 

7.3 

12.4 

1600 

1  Important  because  of  the  additions  of  organic  matter  that  occurs 
thereby. 

2  It  is  estimated  that  of  the  crops  removed  and  fed  or  used  as  bedding, 
only  30  per  cent,  of  the  N,  K20,  CaO  and  S03,  50  per  cent,  of  the  P205 
and  25  per  cent,  of  the  organic  matter  reach  the  soil  as  farm  manure  (par. 
294).  The  crops  removed  carried  about  4000  pounds  of  organic  matter 
to  the  acre. 

3  The  green-manure  is  estimated  as  4000  pounds  of  air-dry  matter 
carrying  100  pounds  of  nitrogen,  which  is  considered  as  fixed  from  the 
air,    This  should  yield  3000  pounds  of  soil  organic  matter. 


558        NATURE  AND  PROPERTIES  OF  SOILS 

320.  The  balance  sheet. — For  convenience  of  compari- 
son, the  data  previously  presented  are  drawn  together  in  a 
single  table  and  presented  below  as  pounds  to  the  acre  an- 
nually. These  figures  are  considered  as  relating  to  the  Dun- 
kirk silty  clay  loam  carrying  a  five-year  rotation  of  maize, 
oats,  wheat,  and  two  years  hay.  It  must  always  be  remem- 
bered that  such  data  are  specifically  applicable  to  only  one 
soil.  Nevertheless  the  practical  deductions  that  may  be  drawn 
are  of  wider  scope. 

Table    CXXXI 

SUMMARY  TABLE  OP  LOSSES  AND  ADDITIONS  THAT   MIGHT  OCCUR 
TO  DUNKIRK  SILTY  CLAY  LOAM  UNDER  A  FIVE-YEAR  ROTA- 
TION.    EXPRESSED  IN  POUNDS  TO  AN  ACRE  A  YEAR. 


Conditions 

N 

40.3 
21.1 
41.1 
20.0 

PA 

K20 

CaO 

SO, 

Organic 
Matter 

Keductions     when     farm 
manure  and  green  ma- 
nure are  not  used  *.  . . . 

Additions  from  farm  ma- 
nure      

43.5 
21.7 
21.7 

174.1 
31.6 
31.6 

370.2 
7v3 
7.3 

84.5 
12.4 
12.4 

1200 
1000 

Additions  from  farm  ma- 
nure and  green-manure 

Additions     using     green- 
manure    

1600 
600 

It  is  immediately  apparent  that  when  farm  manure  and 
green-crops  are  not  utilized,  a  notable  decrease  occurs  in  every 
constituent  cited.  Such  a  system  of  soil  management  must 
reduce  the  productivity  of  the  soil  very  quickly  and 
certainly  is  not  a  rational  scheme  of  soil  and  crop  adjustment. 
Nevertheless,  it  is  the  condition  under  which  much  of  the 
arable  land  is  producing  crops  today. 

When  farm  manure  is  utilized,  even  allowing  for  a  large 

1  Obtained   by    subtracting    the    natural    additions    from    the    normal 


THE  MAINTENANCE  OF  SOIL  FERTILITY     559 

waste  in  its  production  and  handling,  the  organic  matter  is 
almost  maintained  and  the  loss  of  nitrogen  is  met  to  some  ex- 
tent. Under  such  a  system,  the  addition  of  nitrogen  and  of 
mineral  constituents  is  a  problem,  although  same  attention 
should  be  paid  to  the  soil  organic  matter.  Liming  will  be 
necessary  ultimately  if  not  immediately,  while  the  addition 
of  phosphoric  acid  obviously  will  some  day  be  profitable. 
If  acid  phosphate  is  utilized  at  a  normal  rate,  the  sulfur 
losses  that  occur  should  be  very  nearly  counterbalanced. 
Potash,  especially  as  the  soil  under  consideration  is  a  clay 
loam,  will  no  doubt  be  available  for  a  long  period  if  the 
organic  matter  is  adequately  maintained. 

The  use  of  a  green-manure  once  in  the  rotation  in  addition 
to  the  farm  manure  will  adequately  care  for  the  soil  or- 
ganic matter  and  reduce  the  nitrogen  problem  to  a  minor 
position. 

When  animal  products  are  relatively  high  in  price  and 
crop  values  are  low,  stock  farming  will  be  advisable  and  a  sys- 
tem whereby  considerable  farm  manure  will  be  available  may 
be  followed.  It  has  already  been  indicated  that  under  such 
conditions  the  organic  matter,  and  to  a  lesser  degree  the  nitro- 
gen content  of  the  soil,  may  adequately  be  maintained  espe- 
cially if  a  green-manure  is  used  once  in  the  rotation.  Where 
grain  farming  is  necessary,  reliance  must  be  placed  almost 
wholly  on  green-manures  for  the  upkeep  of  the  soil  organic 
matter,  especial  care  being  given  to  the  full  utilization  of  crop 
residues.  According  to  the  data  presented  in  Table  CXXXI, 
such  a  system,  as  far  as  the  nitrogen  and  organic  matter  are 
concerned,  could  be  made  about  as  satisfactory  as  where  farm 
manure  is  available  and  has  the  possibility  and  advantage  of 
considerable  expansion.  Grain  farming  makes  necessary, 
however,  a  more  intensive  and  careful  use  of  mineral  constitu- 
ents. Liming  and  commercial  fertilizers  will,  therefore,  fig- 
ure somewhat  more  prominently  in  grain-growing  than  where 
dairying  or  stock-raising  are  practiced. 


560        NATURE  AND  PROPERTIES  OF  SOILS 

321.  The  maintenance  of  soil  fertility.1 — The  practical 
management  of  a  soil,  whereby  profitable  crops  may  be  grown 
without  materially  reducing  the  fertility  of  the  land  rests 
on  five  fundamental  principles.  The  basic  factors  are:  (1) 
drainage,  (2)  tillage,  (3)  organic  matter,  (4)  lime,  and  (5) 
fertilizers.  Obviously,  the  removal  of  excess  water  depends 
on  adequate  drainage,  while  aeration  and  all  of  the  activities 
that  attend  it  rests  both  on  drainage  and  tillage.  The  upkeep 
of  the  soil  organic  matter  by  the  use  of  crop  roots  and  resi- 
dues, by  farm  manure,  and  by  the  turning  under  of  green- 
crops  has  already  been  emphasized  as  fundamental  to  con- 
tinuous productivity. 

These  factors  are  by  no  means  the  whole  program  of  ra- 
tional soil  management.  Artificial  additions  must  be  made. 
Of  these  lime  is  of  vital  importance.  Calcium  and  magnesium 
are  lost  from  the  soil  in  such  large  amounts  that  outside 
sources  must  be  drawn  on.  Every  arable  soil  will  ultimately 
come  to  the  point  where  liming  will  be  profitable.  Finally,  the 
judicious  use  of  commercial  fertilizers  must  receive  attention. 
The  addition  of  phosphoric  acid  will  probably  be  the  first 
fertilizer  element  to  be  considered  seriously,  especially  in  gen- 
eral farming.  Under  special  conditions  of  soil  and  crop,  nitro- 
gen and  potash  will  also  be  a  part  of  the  program.  The  adap- 
tation of  crops  in  suitable  rotation  to  climate  and  soil,  with 
adequate  attention  to  the  factors  emphasized  above,  are  the 
prime  essentials  of  a  paying  system  of  permanent  soil  pro- 
ductivity. 

1  Hartwell,  B.  L.,  and  Damon,  S.  C,  Six  Years'  Experience  in  Im- 
proving a  Light  Unproductive  Soil;  Jour.  Amer.  Soc.  Agron.,  Vol.  13, 
No.  1,  pp.  37-41,  1921. 

Lipman,  J.  G.,  and  Blair,  A.  W.,  The  Lime  Factor  in  Permanent 
Soil  Improvement.  I.  notations  without  Legumes.  II.  notations  with 
Legumes;  Soil  Sci.,  Vol.  IX,  No.  2,  pp.  83-114,  1921. 


INDEX  OF  AUTHORS 


Aarnio,  B.t  134. 

Abbott,  J.  B.,  347. 

Aberson,  J.  H.,  252. 

Agee,  A.,  362. 

Ageton,  C.  U.,  376. 

Aikman,   C.   M.,    504. 

Allen,  E.  R.,   424. 

Allison,  F.  E.,  387. 

Alway,  F.  J.,  43,  44,  63,   115,   118,   119 

120,    155,    163,    16G,    195,    198, 

199,   221. 
Ames,  J.  W.,  58,  406,  470,  499,   550. 
Amnion,  Georg,   156. 
Appiani,   G.,   73. 
Appleyard,   A.,    158,    248,   252,   253,    257, 

273,  543. 
Ashby,  S.  F.,  424. 
Ashley,    H.    E.,    132,    134,    137. 
Atterburg,  A.,   68,   140,   141,   187. 
Averitt,   S.   D.,   118. 
Aumer,  J.  P.,   545. 

Bancroft,   W.    D.,    127,   129. 

Barker,  P.  B.,  221. 

Barlow,   J.    T.,   358. 

Barrett  Company,   The,   449. 

Baumann,   A.,   106. 

Beattie,  J.   H.,   351,   520,   521. 

Beaumont,  A.  B.,  134,  157. 

Beavers,   J.   C,   499. 

Bennett,  H.  H.,  38,  50,  63,  118. 

Bernard,  A.,  467. 

Bertholot,    M.,    431. 

Bertrand,    G.,    466. 

Bishop,   E.   S.,  120. 

Bizzell,   J.   A.,    179,   207,   251,    252,   284, 

422,   436,   437. 
Blair,   A.   W.,   307,    354,   356,    381,   556, 

560. 
Blanck,   E.,   478. 
Bogue,  R.  H.,  267. 
Bolley,   H.   L„   397. 
Boltz,  G.  E.,  406,  470. 


Boullanger,  E.,  466,  467. 

Bouyoucos,  G.  J.,  153,  154,  155,  159, 
160,  171,  175,  182,  196,  223, 
228,  232,  235,  239,  259,  277, 
280,   281,   286,   287,  291,  356. 

Bradley,  O.  E.,   380. 

Breazeale,  J.  F.,   113,  331,  381. 

Brenchley,  W.   E.,  284,   287. 

Briggs,  L.  J.,  67,  113,  156,  168,  171, 
172,  188,  190,  195,  196,  197, 
277,  278,  380. 

Bright,  J.   W.,   505. 

Briscoe,   C.    F.,    544. 

Brodie,   D.  A.,  499. 

Bronet,   G.,   267. 

Brooks,    W.    P.,    461,    522. 

Broughton,    L.   B.,   377. 

Brown,   B,   E.,   Ill,  422. 

Brown,   C.   F.,  211,  340. 

Brown,  P.  E.,  44,  362,  388,  392,  405, 
406,   421,    424. 

Bryan,  H.,   71. 

Buckingham,   E.,    182,   260. 

Buckman,  H.   O.,   29. 

Buddin,  W.,  414. 

Bunger,  H.,   186. 

Burd,  J.  S.,  280,  322,  325. 

Burdick,    R.    T.,    499. 

Burr,   W.   W.,    194,   221. 

Burton,  E.  F.,  127,  129. 

Caldwell,  J.  S.,   195. 

Call,  L.  E.,  220,  221. 

Cameron,  F.  K.,   113,   141,  283. 

Carr,   R.   H.,   116,   143. 

Carrero,  P.  L.,  299. 

Carter,   E.   G.,  392,   393,  421,   464. 

Cates,  J.  S.,  221. 

Chamberlain,  T.   C,  57. 

Chase,   L.   W.,    144. 

Christensen,  H.  R.,  354. 

Clarke,  F.  W.,  4,   13. 

Clarke,   V.   L.,   155,   198. 


561 


562 


INDEX  OF  AUTHORS 


Coffman,  W.  B.,  190. 

Coleman,  D.  A.,  387,  388. 

Coleman,  L.  C,  420. 

Collins,   S.    H.,    442. 

Comber,  N.   M.,  360. 

Conn,  H.   J.,   388,  389,   505. 

Conn,   H.   W.,   384. 

Conner,   S.   D.,    295,   328,  347,   349,   351, 
457,  461. 

Cook,   R.  C,   273. 
Coppenrath,    E.,   466. 
Cox,  H.  R.,  221. 
Cowles,  A.  H.,  380. 
Crosby,  W.  A.,  36. 
Crowther,   C,    429,    469. 
Cullen,  J.  A.,   464. 
Cummins,  A.  B.,  267. 
Curry,  B.  E.,  267,  380. 
Curtis,  R.  E.,  389. 
Cushman,  A.  S.,  73,  132. 
Czermak,   W.,   143,   296. 

Damon,  S.  C,  381,  476,  560. 

Darbishire,   F.   V.,   414. 

Davidson,  G.,   109. 

Davidson,   J.   B.,    144. 

Davis,    A.    R.,    432. 

Davis,  N.  B.,  137. 

Davis,   R.  O.   E.,   171,  204. 

Davis,  W.  M.,  46. 

Davy,  J.   B.,  340. 
Demolon,  A.,  267,  467. 
Digby,   Kenelm,  442. 

Diller,   J.    S.,   33. 

Dobeneck,  A.  F.,  156. 

Dorrance,   R.    L.,    430. 

Dorsey,  C.  W.,  330,  334,  337,  340. 

Doryland,   C.   J.   T.,   249. 

Duchacek,    F.,    460. 

Dugardin,   M.,   467. 

Duggar,   B.    M.,    432. 

Duley,   F.   L.,   499. 

Dupre,   H.   A.,   169. 

Dyer,  Bernard,   318. 

Eastman,   E.   E.,   204. 
Ehrenberg,  P.,    134,   143. 
Ellett,   W.   B.,    362. 
Elliott,   C.  G.,  210,   213. 
Emerson,  H.   L.,   17,  40,  46. 
Ernest,  A.,   110,  252. 


Failyer,    G.    H.,    77,    268,    279,    321. 

Faure,    L.,    210. 

Feilitaen,  H.  von,  429,  467. 

Fellers,   C.   R.,   387. 

Fippin,   E.   O.,   122,    143,    211,   499,   516, 
519. 

Fisher,  M.  L.,  204. 

Fleischer,  M.,  44. 

Fletcher,   C.   C,   71. 

Floess,   R.,   134. 

Floyd,  B.  F.,  363. 

Flugel,    M.,   478. 

Fraps,  G.   S.,   208,  319,  420,   443,   471. 

Frear,  W.,   362,   376,   377,   516,   524. 

Freckmann,   W.,    186. 

Fred,  E.  B.,  505,  643. 

Fry,  W.   H.,  6,  76,   133,  445,  455. 

Fulmer,   H.   L.,   505. 

Funchess,  M.   J.,   347. 

Gaither,   E.  W.,  58,  499. 

Gainey,   P.   L.,   249,   356,   392,    420,   424. 

Gallagher,   F.   E.,   141,    143,   273. 

Gans,  R.,   265. 

Gedroiz,  K.  K.,  459. 

Gee,   E.   C,   211. 

Georgeson,   C.   C,   239. 

Gerlach,   U.,    272,    306. 

Gilbert,  J.  H.,  180,  443. 

Gile,  P.  L.,  299,   376,  466. 

Gillespie,   L.   J.,   281,   350,   356. 

Glass,   J.   S.,   204. 

Goessman,  C.  A.,  504. 

Gortner,  R.  A.,   116. 

Grandeau,    L.,    115. 

Greaves,  J.   E.,  392,   393,   420,   421,  422, 

432,  433. 
Gustafson,   A.    F.,    124,    220,    242. 
Guthrie,  F.   B.,   336. 

Haberlandt,   H.,   140,   224. 

Hall,   A.   D.,    10,    68,    78,   206,   217,   284, 

287,    294,    305,    421,    433,    442, 

448,    449,    471,    508,    516,    519, 

531,    532. 
Halligan,   J.   E.,   442,   471. 
Harned,    H.    H.,   544. 
Harris,  F.   S.,  328,  334,  336,  340. 
Hart,    E.    B.,    303,    315,    468,    499,    514, 

521. 
Hart,   R.   A.,   211,   340. 
Harter,   L.   L.,   337. 


INDEX  OF  AUTHORS 


563 


Hartwell,  B.  L.,  347,  349,  354,  381,  461, 

476,   560. 
Hasenbaumer,  J.,   466. 
Headden,   W.  P.,   330,  332. 
Heinrich,    R.,    197. 
Hellriegel,  H.,  189,  191,  192. 
Helms,    R.,    336. 
Hendrick,  J.,  78. 
Hibbard,  P.  L.,   342. 
Hildebrandt,  F.  M.,  267. 
Hilgard,    E.    W.,    31,    68,    73,    116,    120, 

155,  162,  330,  340. 
Hill,  H.  H.,  550. 
Hills,  J.  L.,  483. 
Hiltner,  L.,  389. 
Hirst,  C.  T.,  464. 
Hitchcock,  E.  B.,  421. 
Hoagland,    D.    R.,    279,    280,    281,    284, 

285,   323,   350. 
Hoffman,   C,   461. 
Hopkins,  C.  G.,  122,  355,  362,  437,  466, 

516,   533,   539,   540,   545. 
Houston,   H.  A.,   116. 
Howard,  L.  P.,  353,  354. 
Hubbard,  P.,  73. 
Hudelson,  R.  R.,  362. 
Hudig,  J.,  429. 
Humphreys,  W.  J.,  53. 
Hunt,  T.  F.,  472,  533. 
Hurst,   L.  A.,  350,  356. 
Hutchinson,  C.  M.,  545. 
Hutchinson,    H.    B.,    108,    355,    387,    402, 

411,    414,    415,    424,    447,    450, 

544. 

Ingle,  Herbert,   100. 
Isham,  R.  M.,  332. 
Israelsen,  O.  W.,  92,  93,  163. 

Jaffrey,  J.  A.,   211. 
Jensen,   O.  A.,    545. 
Jodidi,  S.  L.,  106,  248. 
Joffe,  J.  S.,  351,  356. 
Johnson,  H.  W.,  405. 
Johnson,  S.  W.,  249. 
Jones,  C.   H.,   355,   483. 
Jones,  S.  C,  499. 
Juritz,  C.  F.,   430. 

Karraker,  P.  E.,  163,  171,  358. 
Kaserer,  H.,  416. 


Kearney,  T.  H.,  337. 

Keen,  B.  A.,  151. 

Kellerman,   K.  F.,   424. 

Kellner,  O.,  450. 

Kellogg,   E.   H.,   405. 

Kellogg,  J.  W.,  365,  379. 

Kelley,   N.  P.,   107,   267,  347,   415,   421, 

450. 
Kelly,  M.  P.,  466. 
Kiesselbach,  T.  A.,  188. 
King,    F.    H.,    94,    145,    163,    172,    176, 

189,    210,    230,    241,    242,    282, 

284,  422. 
Kinnison,  O.  S.,  140. 
Klippart,  J.  H.,  210. 
Knox,   J.,    450. 
Knox,  W.  H.,  355. 
Knudson,  L.,  402. 
Koch,  G.  P.,  388. 
Konig,  J.,  466. 
Kopecky,  J.,  179. 
Kopeloff,   N.,   377,   387,   388,  413. 
Koppers  Company,  The,  449. 
Kratzman,   E.,   347. 
Krusekopf,  H.   H.,  362. 
Krzymowski,   R.,    187. 


Lang,  0.,  228,  232. 

Lapham,  M.  H.,  171. 

Lathrop,  E.  C,  107,  410. 

Latshaw,  W.  L.,  437. 

Lau,  E.,  248. 

Lawes,  J.  B.,  180,  189,  443. 

Leather,  J.  W.,  190. 

Leidigh,  A.  H.,  211. 

Lemmermann,  O.,   551. 

Liebig,  J.  Justus  von,  443. 

Lipman,  C.  B.,  32,  158,  278,  376. 

Lipman,  J.  G.,  307,  342,  381,  384,   403, 

406,    431,    433,    436,    508,    537, 

556,  560. 
Loew,   O.,   376,   415. 
LShnis,   F.,   433. 
Loughridge,    R.    H.,    78,    122,    156,    198, 

337. 
Lugner,   I.,   429. 
Lyon,    T.    L.,    179,    181,    207,    251,    252, 

284,    297,    422,    436,    437,    495, 

541,  542. 
Lynde,  O.  J.,  169. 
Lynde,   H.   M.,   211. 


564 


INDEX  OF  AUTHORS 


370, 


Maclntire,    W.    H.,    181,    250,    345, 

371,  422. 
MacLennan,  K.,  355. 
Martin,  J.  C,  280,  285. 
Martin,  L.  M.,  46. 
Martin,  T.  L.,  541,  545,  547. 
Martin,  W.   H.,  351. 
Marchal,  E.,  335,   413. 
Marshall,  O.   E.,  384. 
Massey,  A.  B.,  109. 
May,  D.  W.,  466. 
Mayer,  A.,  200. 
Maynard,  L.  A.,  545. 
Maze,   P.,   402,   478. 
McBeth,  I.  G.,  267,  389. 
McBride,  F.  W.,   116. 
McOall,  A.  G.,  267,  278. 
McCaughey,  W.  G.,  5,  36,  76. 
McCool,  M.   M.,  362. 
McDole,  G.  R.,  118,   166. 
McGeorge,  W.  T.,  78,  79,  273. 
McLane,  J.  W.,  168,  277. 
McLean,  H.  C,  388. 
McMillar,  P.  R.,  380. 
Merrill,  G.  P.,  3,   17,  32,  36,  38,  265. 
Middleton,  H.  E.,  133. 
Miles,  M.,  210. 
Millar,  C.  E.,  362. 
,  448. 
190,  194. 
362. 
Miller,  N.  H.  J.,  10,  108,  402,  411,  415, 

447,   450,   469. 
Milligan,  S.,  545. 
Miner,   H.   L.,   483. 
Minges,  G.  A.,  421. 
Mirasol,  J.  J.,  347,  349. 
Mitscherlich,   E.   A.,   134,   140,   186,   284, 

477. 
Miyake,   K.,  347. 
Molisch,  H.,  296. 
Montgomery,     E.     G.,     188,     190,     191, 

192. 
Mooers,  C.  A.,  181,  362. 
Moore,  C.  J.,  133. 
Morgan,  J.  F.,  278,  281,  282. 
Morrow,  C.  A.,  99,  105. 
Morse,  F.   W.,   122,   267,   380,   449. 
Morse,  W.  J.,  465. 

Mosier,  J.  G.,  124,  204,  219,  220,  242. 
Murray,  T.  J.,  427,  509. 
Mulder,  T.  J.,  105. 


Miller,  B.  L., 
Miller,  E.  C. 
Miller,  M.  F. 


Neller,  J.  R.,  256,  465. 
Niklas,   H.,    127. 
Norton,  T.  H.,  450. 

Ogg,  W.  J.,  78. 
Oliver  Plow  Book,   144. 
Olsen,  C,  351. 
Osborne,  T.  B.,  68,  70. 
Osugi,  S.,  346,  349. 
Owen,  W.  L.,  256. 

Pantanelli,   E.,  284. 

Parker,  E.  G.,  267,  269,  271. 

Parker,   F.   W.,   278. 

Parks,  J.,   242. 

Parsons,  L.  J.,  210. 

Patten,  H.    E.,    154,   232,   235,   237,    263, 

273. 
Patterson,  J.  W.,  420. 
Peake,  W.  A.,  113. 
Peck,  E.  L.,   469. 

Pember,  F.  R.,   347,   349,  354,  381,   461. 
Penny,  C.  L.,  537,  540. 
Peters,  E.,  266,  271. 
Peterson,    W.    H.,    303,    315,    331,    332, 

468. 
Pettit,  J.  H.,  355. 
Pfeiffer,  Th.,  478. 
Pick,  H.,  134. 
Pickel,  G.  M.,  44. 
Pieters,  A.  J.,  537. 
Piper,  C.  V.,  537. 
Pirsson,  L.  V.,  3,  46. 
Pitman,  D.  W.,  336. 
Pitra,  J.,  460. 
Plummer,  J.  K.,  5,  6,  76,  251,  256,  356, 

373,   392,   420. 
Potter,  R.  S.,  251,  315. 
Pranke,  E.  J.,  451. 
Prescott,  J.  A.,   263. 
Prianischnikov,  D.,  459,  460. 
Prince,  A.   L.,   354,  356. 
Puchner,  H.,  78,  141. 
Pugh,  E.,  443. 

Rahn,  Otto,  392. 
Ramann,  E.,  127,  259. 
Ramser,   C.   E.,   204. 
Ramsower,  H.  C,  145. 
Rather,  J.  B.,  113. 
Ravin,  P.,  402. 
Reed,  H.  S.,  109,  297. 


INDEX  OF  AUTHORS 


565 


Reid,   F.  R.,   347,  466. 

Reimer,  F.  0.,   467. 

Rice,  F.   E.,  346,  349. 

Richards,  E.  H.,   429. 

Richmond,  T.  E.,  406. 

Roberts,  I.  P.,  504,  514,  519,  521. 

Robbins,  W.  J.,  109. 

Robbins,  W.  W.,  386. 

Robinson,  C.   S.,  44. 

Robinson,  F.  W.,  539. 

Robinson,  G.  W.,  78. 

Robinson,   W.  O.,   5,   13,   14,   86,  41,   52, 

63,  77,  118,  466. 
Rodewald,   H.,   134. 
Ross,  W.  H.,  464,  466. 
Rost,  C.  O.,   63,   118,  119,   315. 
Ruprecht,  R.  W.,  347,  449. 
Russell,   E.  J.,   9,   68,  78,   158,  248,  252, 

253,    257,    273,    387,    414,    424, 

429,   471,   543. 
Russell,  I.  C,  46. 
Ruston,  A.  G.,  429,  469. 

Sachs,  J.,  296. 

Sachs,  W.  H.,  466. 

Sackett,  W.  G.,  331,  412. 

Salisbury,  R.  D.,  52,  57. 

Salter,  R.  M.,  116. 

Saussure,  Theodore  de,  442. 

Schantz,  H.  L.,  156,  188,  190,  195,  196, 

197. 
Schollenberger,  C.  J.,  315,  354. 
Schone,  E.,  73. 
Schreiner,    O.,    105,    107,    108,    109,    111, 

268,  279,  297,  321. 
Schulze,  F.,  322. 
Schutt,  M.  A.,  507,  519. 
Seelhorst,  C,  von,   186,   187,   192. 
Sewell,  M.  C.,  144,  220,  221. 
Sharp,   L.  T.,   281,   350. 
Shaw,  O.   F.,   92. 
Shedd,  O.   M.,   325,  406,   467. 
Sherman,  J.  M.,  387. 
Shorey,  F.  C,  76,  105,  107,  362. 
Shutt,   F.   T.,   430,   539. 
Singewald,  J.   N.,  448. 
Skinner,  J.  J.,   108,  109,  347,  351,  447, 

466. 
Slosson,  E.  E„  450. 
Smalley,  H.  R.,  44,  347. 
Smith,  Alfred,  89. 
Smith,  C.  D.,  539. 


Smith,  O.  C,  116. 

Snyder,  H.,  122,  284,  317,  545. 

Snyder,  R.  S.,  251,  313,  315. 

Spillman,  W.  J.,  537. 

Spurway,  C.  H.,   287. 

Stephenson,  R.  E.,  353,  354. 

Stevenson,  W.  H.,  44. 

Stewart,  C.  F.,  45. 

Stewart,  G.  R.,  279,   280,   284,  323. 

Stewart,    R.,    331,    332,    376,    381,    404, 

420,   422,   470. 
Stoddard,  C.  W.,  100. 
Stoklasa,  J.,  110,  252,  255,  256,  460. 
Storer,  F.  H.,  504,  537. 
Stormer,  K.,  389. 
Stremme,  H.,  134. 
Strowd,  W.  H.,  435. 
Sullivan,  E.  C.,  267,  270. 
Sullivan,  M.   X.,  107,  466. 
Swanson,  C.  O.,  437. 
Sweetser,  W.  S.,  511. 
Swezey,  G.  D.,  242,  260. 
Tacke,  Br.,  355. 
Tartar,  H.  V.,  467. 
Tarr,  R.  S.,  46. 
Taylor,  W.  W.,  127. 
Tempany,  H.  A.,  134. 
Temple,  J.  C.,  421. 
Thatcher,  R.  W.,   100,  122,  127. 
Thomas,  W.,  376,  377. 
Thompson,    H.    C.,    44. 
Thorne,   O.   E.,   375,   382,   454,  461,   462, 

499,    504,    507,    511,    513,    514, 

516,    519,    521,    525,    526,    528, 

529,   532,   533. 
Tottingham,  W.  E.,  461,  514. 
Trieschmann,  J.  E.,  469. 
Trnka,  R.,  92. 
True,  R.  H.,  300,  346,  348. 
Truog,  E.,  348,  359. 
Turpin,  H.  W.,  252. 


Ulrich,  R.,   232,   5 
Underwood,  T.  M. 


284,  287. 


Vageler,  P.,  134. 

Van  Bemmelen,  J.  M.,  36,  106,  132,  265, 

266,  270. 
Van   Slyke,    L.    L.,    442,    471,    501,    503, 

514. 
Van  Suchtelen,  F.  H.  H.,  278. 
Veitch,  F.  P.,  317,  355. 


566 


INDEX  OF  AUTHORS 


Voelcker,  A.,  519,  532. 
Von  Englen,  O.  D.,   58. 
Voorhees,  E.  B.,  431,  504. 
Vrooraan,  C,  439. 

Waggaman,   W.   H.,    263,   455,   456,    461, 

464. 
Wagner,  F.,  239. 
Wagner,  H.,  477. 
Waksman,    S.    A.,    387,    388,    389,    392, 

413. 
Walker,  S.  S.,  51,  118. 
Walters,  E.  H.,  107. 
Warington,  R.,   113,  132,   144,   180,   182, 

265,   303,   426. 
Warner,  H.  W.,  406. 
Warren,  G.  M.,  210. 
Watson,  G.  C,  514. 
Way,  J.  T„  132,  264. 
Waynick,  D.  D.,  113. 
Weaver,   F.   P.,  499. 
Weir,  W.  W.,  362. 
Welitschkowsky,  D.,  von,  176. 
Wells,  A.  A.,  248. 
Wills,  C.  F.,  116. 
Westerman,  F.,   433. 
Whitbeck,  R.  H.,  58. 
Whitney,  M.,  81,  83,  85,  89. 
Whisenand,  J.  W.,  520. 


White,   J.    W.,    351,   353,    365,    377,    421, 

449,  550. 
Whiting,  A.  L.,  545. 
Whitson,  A.  R.,  44,  204,  362,  422. 
Wiancko,  A.  T.,  365,  381,  461,  499. 
Widtsoe,  J.   A.,   172,   186,   187,   190,   192. 
Wiegner,  G.,  127,  265,  270. 
Wiley,  H.  W.,  73,   112,  114,   115,  314. 
Williams,   C.    B.,   49,   52,    118. 
Williams,  H.  F.,  5. 

Wilson,  B.  D.,  11,  374,  404,  430,  437. 
Wilson,  G.   W.,   388. 
Wilson,  J.   K.,   297. 
Wing,  H.  H.,  514. 
Winogradsky,  S.,  431. 
Wolkoff,  M.  I.,  131. 
Wollny,    E.,     110,    163,    171,    176,     189, 

200,   228,   230,  245. 
Wood,  T.  B.,   516. 
Woodward,  S.   M.,  210. 
Wright,  R.  C,  541. 
Wyatt,  F.  A.,  363,  376,  381. 
Wyckoff,  M.  I.,  267. 

Yarnell,   D.   L.,   211. 
Yoder,   P.  A.,  73. 
Young,  G.  J.,  464. 
Young,  H.  J.,  221. 

Zzigmondy,  R.,   127. 


INDEX  OF  SUBJECT  MATTER 


Ability  of  plants  to  grow  on  poor  soils, 

299. 
Abrasion  defined,  18. 
Absorption  by  litter,   521. 
Absorption,  by  soils  explained,  263. 

capacity   of   soils   to   retain   nitrates, 
321. 

due  to  soil  colloids,   265. 

effect  of  on  soil  acidity,   352. 

effect  of  texture  on,    267. 

importance  of  in  soils,   273. 

of  litter  in  stable,  521. 

selective  by  soils,  nature  of,  269. 

selective  by  soils,  types  of,   269. 
Absorption  by  soils,  capacity  for,  266. 

causes  of,  264. 

defined,  263. 

importance  of,  273. 

influence  of  time  on,   269. 

law  of,   269. 

relation  to  acidity,  274. 

relation  to  the  soil  solution,  276. 

selective,    269. 

types  of,  263. 
Absorption    of    solar   insolation,    as   influ- 
enced by  atmosphere,  226. 

as  influenced  by  color,  228. 

as  influenced  by  slope,  229. 

as  influenced  by  soil,  226. 
Absorptive    capacity    of    different    crops, 

301. 
Acid  phosphate,   456. 

changes  in  soil,  457. 

character,  456. 

compared  with   rock  phosphate,    458. 

composition,   456. 

manufacture,   456. 

reinforcement  of  manure  with,  528. 
Acidity,  as  influenced  by  absorption,  274. 

development  of  by  hydrolysis,  348. 

production    of    by    selective    absorp- 
tion, 270. 

soil,  nature  of,  345. 
Acids,  production  of  by  plant  roots,  296. 
Actinomyces  in  soils,  character  of,  389. 


Actinomyces  in  soils,  importance  of,  389. 

number  of,  289. 
Addition  of  nutrients  to  soil,  556. 
Additions  to   and  losses  from   soil   under 

various  types  of  farming,  558. 
Adobe,  inportance  of,  64. 

origin  of,  64. 

wind  formation  of,  21. 
jEolian  soils,  adobe,  64. 

loess,   61. 

sand  dunes,   64. 

volcanic  dust,  65. 
Aeration    of    soil,    effect   on    nitrification, 
418. 

importance  in  soil,  256. 

influence  on  bacteria  in  soils,  393. 
Agglutination  of  colloids,   131. 
Agricultural  lime,   defined,   363. 

forms  of,  363. 
Agricultural  value  of  farm  manure,   513. 
Air  of  the  soil,  carbon  dioxide  of,   250. 

composition  of,    247,    248. 

composition  data,    248,    250. 

effect  of  oxidation  on,   254. 

general  characteristics  of,   247. 

importance  of  oxygen  in,   256. 

practical  modification  of,  261. 

movement  of,   258. 

types  of,  249. 

volume  of,   257. 
Alkali,  black,  329. 

composition  of,  329. 

conditions  affecting  influence  of,  338. 

control  of,   343. 

control  of  by  means  of  gypsum,  342. 

effect  of   concentration   of   on   crops, 
337. 

effect  on  crops,  334. 

effect  on  soil  organisms,   335. 

eradication  of,   341. 

eradication  of  by  means  of  drainage, 
341. 

influence  on  nitrification,  421. 

in  river  water,  332. 

in  irrigation  water,  334. 


567 


568 


INDEX  OF  SUBJECT  MATTER 


Alkali,   origin  of,  331. 

resistance  of  crops  to,  data  on,  338. 

rise    of    as    influenced    by    irrigation, 
339. 

white,  329. 
Alkali  lands,  handling  of,   340. 
Alkali  salts,  listed,  330. 
Alkali  soils,   defined,   328. 

importance  of,  328. 
Alkali  spots,   nature  of,   332. 
Alkali    tolerance    by    plants,    factors    of, 

336. 
Alkali  vegetation,  340. 
Alluvial  fans,  47. 
Alluvial    soils,    chemical    composition,    49. 

classified,  46. 

deltas,  47. 

fans,  47. 

flood  plain,   47. 

importance  of,    49. 

origin  of,  46. 
Aluminum,  hydrolysis  of  in  soil,   348. 

relation  of  to  soil  acidity,  347. 

relation    to    the    reversion    of    acid 
phosphate,   457. 
Alunite  as  a  fertilizer,  465. 
Amino  acids  defined,   106. 

in  farm  manure,  510. 
Amides  defined,   106. 
Ammonia  in  rain  water,  data  on,  429. 
Ammonification,  conditions  for,   414. 

influence  of  protozoa  on,  387. 

nature  of,  412. 

organisms  of,  413. 

products  of,  413. 

reactions  of,   414. 
Ammonifying    efficiency    of    soil,    determi- 
nation of,  414. 
Ammonium    salts,    utilization    by    higher 

plants,   415,  450. 
Ammonium  sulfate,  changes  in  soil,   449. 

character  of,    449. 

composition  of,   449. 

source  of,   449. 
Amounts  of  fertilizer  to  apply,  492. 
Amounts  of  lime  to  apply,  368. 
Analysis  of  plant  tissue,  method  of,   102. 
Analysis  of  soil,  bulk,  311,  314. 

carbon  in,   113,    114. 

extraction,    dilute  acids,    317. 

extraction,  strong  acids,  316. 

extraction,  with  water,   319. 


Analysis  of  soil,  humus,  115. 

lime  requirement  of,  355. 

minerological,    76. 

nitrogen  in,   311. 

organic  matter  of,  115. 

value  of,  323,   326. 
Apatite  in  soil,  6. 

Application    of    farm    manure,    amounts, 
526. 

evenness,    526. 

incorporation  in  soil,   526. 
Arid   soils,   biological   activity  in,    32. 

chemical   analysis  of,   31. 

humus  content  of,   120. 
Assimilation    of    nitrates    by    soil    organ- 
isms, 426. 

importance  of,  428. 
Available  water  in  soil,   198. 
Availability  of   nitrogen   fertilizers,   454. 

of  phosphate  fertilizers,  458. 
Azofication,     amount    of     nitrogen    fixed, 
433. 

energy   for,  432. 

organisms  of,  432. 
Azotobacter  ehroococcum   in   soil,   431. 

B.    Radicicola,   amount   of    nitrogen   fixed 
by,   437. 

availability  of  nitrogen  fixed  by,  437. 

function  of,   434. 

importance  of,  436. 

inoculation   of   the  soil,   methods  of, 
439. 

nature  of  organism,  435. 

nodules  of,  434. 

relation  to  host  plant,  435. 

strains   of,    434. 
Bacteria,    decomposition   of   organic   mat- 
ter by,   103. 

increase  of  in  frozen  soil,  394. 

influence  on  aeration  in  soils,   393. 

injurious  to  higher  plants,    396. 

method  of  counting  in  soil,  392. 

multiplication  of,   391. 

production  of  carbon  dioxide  by,  252. 

relation  of  to  alkali,  332. 

relation  to  liming,  395. 

relation  of  moisture  to,   393. 

relation    to    organic    matter    in    soil, 
394. 

relation  to  soil  acidity,  395. 

relation  to  soil  temperature,  394. 


INDEX  OF  SUBJECT  MATTER 


569 


Bacteria,  shape  of,  391. 

seasonal  flora,   394. 

spore  formation  by,   391. 
Bacteria  in  soils,  character  of,  390. 

determination  of  numbers  of,   392. 

factors  affecting  growth  of,  393. 

influence  of  green  manures  on,  544. 

numbers  of,  392. 

position  in  soil,   391. 

production  of  enzymes  by,   390. 

size  of,  391. 
Bacterial    activity,    measured    by    carbon 

dioxide  produced,    256. 
Bacterial     growth,     conditions     affecting, 

393. 
Bases,    substitution   of  in  soils,    271. 

those    used    to    correct    soil    acidity, 
362. 

toxic  nature  of  in  acid  soils,  346. 
Basic  exchange,   270. 

influence  on  drainage  water,  305. 
Basic  slag,  changes  in  soil,  458. 

character  of,  458. 

composition  of,  457. 

source  of,  457. 
Beaker  method  of  mechanical  soil  analy- 
sis, 69. 
Biological   cycles  of  the  soil,   importance 
of,  398. 

names   of,    399. 

nature  of,  398. 
Biological  effects  of  lime  on  soil,  371. 
Bog  lime,  nature  of,  45. 
Bomb    method    for    determining    soil    or- 
ganic matter,  114. 
Bone  phosphate,   changes  in  soil,  455. 

character,    454. 

composition,    454. 

source,  454. 
Brands  of  fertilizers,   478. 
Bromberg  soil  tanks,   data  from,   306. 
Brownian   movement,    explained,    128. 
Bucher  method  of  fixing  nitrogen,  453. 
Bulk  analysis  of  soils,   carbon  and  nitro- 
gen,  311. 

mineral  constituents,  314. 
Burned  lime,  364. 

Calcium,  amount  in  soils,  13. 
forms  of  in  soil,   11. 
importance    of     in    fertility    evalua- 
tions,  324. 


Calcium,  in  soil  minerals,   6. 

lack   of    in    relation    to    soil    acidity, 
348. 

loss  of  from  soil,  307,  370,  555. 

of  di-silicate  as  an  amendment,   380. 

relation    to    reversion    of    acid    phos- 
phate, 457. 

use  of  as  lime,  363. 
Calcium  cyanamid,  change  in  soil,  452. 

character  of,   452. 

composition  of,   452. 

manufacture  of,  451. 
Calcium    and    magnesium    ratio    in    soils, 

375. 
Calcium    in    gypsum    as    an    amendment, 

379. 
Calcium  losses,  Bromberg  lysimeters,  306. 

from  Cornell  soils,  307. 
Calcium  nitrate,  character  of,  452. 

composition  of,  452. 

manufacture  of,  452. 
Capillary-absorbed  water,    defined,    196. 
Capillary  capacity  of  soils,  factors  affect- 
ing, 163. 
Capillary  film,  thickness  of  and  effect  on 

capillary  movement,    171. 
Capillary   movement   of   soil   water,    data 
on  rate,  174. 

effect  of  structure  on,  174. 

effect  of  texture  on,  173. 

factors  affecting,  170. 

explained,    168. 

influence  of  film  thickness,   171. 

relation  to  soil  mulch,   175. 

role  in  supplying  plants  with  water, 
193. 
Capillary  pull  of  soils,  data  on,  169. 

determination  of,   168. 
Capillary  water  of  soil,   amounts   in  soil 
columns,   165. 

colloidal  control  of,  159. 

defined,   159. 

determination  of  amount,  161. 

kinds   of,    159. 

position  of  inter  film,   160. 

surface  tension  control,   159. 
Carbide  method  of  fixing  nitrogen,  451. 
Carbon,  cycle  of  in  soil,  399. 

determination  of  in  soil,  113. 

gain  of  by  green  manures,  539. 

in  Cornell   drainage  water,   402. 

in  organic  matter,  113. 


570 


INDEX  OF  SUBJECT  MATTER 


Carbon,  loss  from  the  soil,  data  of,  402. 

loss  of  from  soil,  555. 

use    of    organic    carbon    by    higher 
plants,  402. 
Carbon  cycle  of  the  soil,  loss  of  carbon 
from,  400. 

nature  of,  399. 

organisms  of,  399. 

products  of,  400. 
Carbon    dioxide,    a    measure   of   bacterial 
activity,  256. 

from  decaying  manure,   509. 

from  lime,   369. 

function  in  soil,  255. 

in  atmospheric  air,  data,   110. 

in  soil  air,  data,  110. 

influence  on  nitrification,  256,  419. 

relation  to  mineral  cycle  of  soil,  408. 

of  soil  air,   250. 

of  soil  air,  influence  of  farm  manure 
on,  254. 

of  soil  air,  influence  of  organic  mat- 
ter on,  253. 

production  of,  110. 

produced  by  bacteria  in  soil,  252. 

produced  by  plant  roots,  252,  295. 

source  of  in  soil  air,  251,  400. 
Carbonated  lime,  365. 
Carbonation,    influence  in  soil   formation, 

26. 
Carbonized  materials   in  soil,   importance 
of,   112. 

nature  of,  111. 
Castor  pomace,  composition  of,   446. 
Catalytic   fertilizers,    466. 
Catalyst  defined,  103,   135. 
Cell  sap,   nature  of  in  relation  to  plant 

absorption,   300. 
Centrifugal   mechanical   analysis  of  soils, 

71. 
Character  of  soil  particles,  69. 
Chemical  absorption  by  soils,  263. 
Chemical    analysis,    alluvial    and    upland 
soils,   49. 

arid  and  humid  soils,   31. 

bulk  and  extraction  methods,  311. 

by  digestion  with  strong  acids,  316. 

by  water  extraction,  319. 

glacial  soils,   57. 

granite  soil,  33. 

importance    in    fertility    evaluation, 


soils, 


311. 


326. 


Chemical  analysis,   limestone  soil,   33. 
loess  soils,   63. 
marine  soils,   52. 
of  alkaline  river  water,  334. 
of  Cornell   soils,    325. 
of  good  and  poor  Ohio  soils,  326. 
of  Minnesota  soils,  316. 
of  Minnesota     and     Maryland 

317. 
of  soil,  popular  conception  of, 
of  soil  separates,   78,  79. 
peat  and  muck,   44. 
residual  soils,  41,  52,  57. 
resume  as  to  value  of,  326. 
value  as  shown  by  actual  data, 
with  weak  acids,  317. 
Chemical   composition  of   soils,   compared 

to  lithosphere,   13. 
Chemical    composition    of    soil    separates, 

78,  79. 
Chemical  effects  of  lime  on  soil,   371. 
Chromic    acid    method    for   determination 

of  soil  organic  matter,   113. 
Chile  salt  petre,  source  and  character  of, 

448. 
Classification    of    methods    of    mechanical 

analysis,  72. 
Classification  of  soils,   geological,   38. 

for  soil  survey,  85. 
Classification  of  soil  particles,   Bureau  of 

Soils,  67. 
Classification  of  soil  particles  other  than 

Bureau  of  Soils,   68. 
Climate,   effect  on  transpiration,   191. 
relation  of  to  soil  formation,  30. 
Clostridium  pastorianum  in  soil,  431. 
Coastal  plain  soils,   chemical  composition 

of,  52. 
Cohesion,   cause  of  in  soils,   136. 

defined,   136. 
Colloidal  materials,  properties  of,   130. 

in  soils,   265. 
Colloidal    matter,    absorptive    power    for 

water,   153. 
Colloidal  matter,    influence   on  soil   prop- 
erties,  135. 
Colloidal    matter    in    soils,    influence    on 
structure,  137. 
estimation  of,   134. 
generation  of,  132. 
resume  of,  138. 
Colloidal  particles,  size  of,  128. 


INDEX  OF  SUBJECT  MATTER 


571 


Colloidal  state,  defined,  127. 

defined  briefly,   75. 

electrical  condition  of,   131. 

examples  of,  130. 

phases   of,    129. 

practical   importance  of,   135. 

relation  of  to  granulation,    142. 
Colloids  and  crystalloids,    129. 
Color  of  soil,  compounds  of,  36,  37. 

influence  on  absorption  of  insulation, 
228. 

nature  of,  36. 
Colluvial  soils,  origin  and  nature,  45. 
Commercial   fertilizer,   amounts   to  apply, 
492. 

development  of  use,   442. 

used  for  their  nitrogen,  444. 

used   for  their  phosphorus,    454. 

used  for  their  potash,  462. 

used   in   United   States,   444. 
Commercial  value  of  farm  manure,  512. 
Composition  of  average  soil,    12. 

of  cow  manure,    501. 

of  drainage  water,  304. 

of  farm  manure,   average,   504. 

of  horse  manure,   501. 

of  muck  and  peat,    44. 

of  plant  tissue,   100. 

of  sheep  manure,  501. 

of  swine  manure,   501. 
Composts,  use  of  manure  in,  530. 
Composts  of  sulfur,  406. 
Conductivity    of    heat,     measurement    of, 
235. 

formula   for,    236. 
Conductivity   coefficients   of  various  soils, 

236. 
Conductivity  of  various  soils,  235. 
Conduction,    loss    of    heat    from    soil    by, 

240. 
Conduction    of   heat    in    soils,    factors   af- 
fecting, 235. 

nature  of,  234. 
Constituents    of    soil,    organic    and    inor- 
ganic, 2. 
Control  of  alkali  in  soils,  343. 

of  evaporation,  218. 

of  soil  air,  261. 

of  soil  temperature,  244. 
Conservation  of  soil  moisture,   219. 
Conversion   factors   for  lime,    367. 
Convection   of   heat   in   soil,   238. 


Correction    of    soil    acidity,    bases    useful 

for,    363. 
Corrosion  defined,   18. 
Cotton  and  tobacco,   influence  of  manure 

on,  535. 
Cotton  seed  meal,  composition  of,  446. 
Cover  crops,  influence  on  nitrates  of  soil, 

541. 
Crop     resistance     to     alkali,     generalized 
table  of,   338. 
in  pounds  per  acre,  338. 
Crop  residues,   to  maintain  organic  mat- 
ter, 124. 
Crop   rotation,    relation   of  to  green  ma- 
nuring, 552. 
Crops,  absorptive  capacity  of,  301. 
amounts  of  fertilizers  for,  493. 
bacteria  injurious  to,  396. 
detrimental  influence  of  nitrogen  on, 

473. 
eftect    of    calcium     and    magnesium 

ratio  on,  376. 
effect    of    concentration    of    salts   on, 

337. 
effect    of    on    conservation    of    plant 

nutriants,   308. 
fertilizer  formula  for,  491. 
for  green  manures,   546. 
fungi  injurious  to,   396. 
influence  of  green  manures  on,  547. 
influence  of  manure  on,  532. 
influence  of  nitrogen  on,  472. 
influence  of  phosphorus  on,  474. 
influence  of  potassium  on,  475. 
injurious  effect  of  soil  organisms  on, 

396. 
quantities    of   nutrients    removed   by, 

303. 
removal  of  nutrients  by,   555. 
removal  of  sulfur  by,  404. 
removal    of    sulfur    and    prohphorus 

by,  468. 
response  to  lime,  372. 
systems  of  fertilizing,   496. 
Crushers,   action  of,   148. 
Cultivation,  implements  for,  147. 

importance  of,   219. 
Cultivators,  action  of,  147. 
Cumulose    soils,    agricultural    importance, 
43. 
location  of,  42. 
origin,  42. 


572 


INDEX  OF  SUBJECT  MATTER 


Decay    and    decomposition    defined,    103, 
410. 
and  putrefaction,    in    nitrogen    cycle, 

410. 
and  putrefaction,  organisms  of,   411. 
effect  on  soil  temperature,  239. 
of  farm  manure,  importance  of,  511. 
of  green   manure,    influence   on    lime 

and  phosphorus,    545. 
of  green  manure,  influence  on  nitrate 

accumulation,    543. 
of  green  manure,   influence  on  nitri- 
fication, 544. 
of  green  manure,  stages  of,  542. 
of   organic   matter   in   soil,    products 
of,  110. 
Decomposition,  defined,   17. 

of  organic  matter  in  soil,   103. 
Delta  soils,  47. 

Denitrification,  use  of  term,   426. 
Deoxidation,    influence   in   soil   formation, 

24. 
Deposition,  its  relation  to  soil  formation, 
16. 
relation  to  lime  requirement,   356. 
Depression     of     the     freezing     point,     a 
method    of    studying    soil    solu- 
tions, 280. 
Determination   of  soil   humus,    115. 
Determination     of     soil     organic     matter, 
bomb  method,   114. 
chromic  acid  method,  113. 
loss  on  ignition,   112. 
Di-calcium  silicate  as  a  soil  amendment, 

380. 
Diffusion,   differential  into  plants,  292. 
of  nutrients  into  plants,   291. 
of  soil  air,  260. 
Diminishing  returns,  law  of,   493. 
Disc  plow,  influence  on  soil,  146. 
Disintegration,   defined,    17. 
Dissociation,    defined,    270. 
Drainage    and     evaporation    at    Rotham- 

sted,  217. 
Drainage,   importance  of,   210. 
influence  of,   210. 
loss  of  sulfur  by,  404. 
nutrient  losses  from  soil,  555. 
qualitative  composition   of  water  of, 

304. 
quantitative    composition     of    water 
of,  304. 


Drainage,  use  of  in  eradication  of  alkali, 
341. 

usual  type  of,  212. 
Drainage  water,  carbon  in,  402. 

composition  data  of,   305. 

composition  of,  at  Bromberg,  272. 

importance  of  study,    178. 

qualitative  composition  of,   304. 

quantitative   composition   of,    304. 
Dried  blood,  changes  in  soil,  445. 

character,  445. 

composition,  445. 

source,  445. 
Earth  worms,   importance  of,   385. 
Earth's  crust,   minerals  in,  4. 
Electric   arc   method   of    fixing    nitrogen, 

452. 
Electrolyte,   defined,   130. 

effect  on  colloids,   131. 
Element  in  the  minimum,   476. 
Energy     necessary     for     evaporation     of 

water,   241. 
Energy,   wave  length  of,   225. 
Enzymes,  action  of,   390. 

defined,  103,  390. 

importance  in  soils,   103. 
Eradication   of   alkali   from   soils,    341. 

types  of,  341. 
Erosion  defined,    18. 

relation  to  soil  movement,  16. 
Erosion  of  soil,   control  of,   204. 

types  of,  205. 
Evaluation  of  farm  manure,   512. 
Evaporation    and    drainage    at    Rotham- 

sted,  217. 
Evaporation  of  soil  moisture,   control  of, 
218. 

energy  necessary  for,   241. 

influence  of  on  soil  heat,  241. 

loss  of  soil  water  by,  216. 

water   influenced  by,   182. 
Exfoliation  in   soil  formation,   21. 
Exhaustion  of  soil,   discussion  of,   309. 

possibility  of,  308. 
Exosmosis,  nature  of,  290. 
Extraction    of    soils,     with    concentrated 
acii 

with  dil  .      tcid*   317. 

with  wa  ■  ..  319. 

with  wa  ve  extractions,  322. 

Exudates,    e  by    plant    roots, 

29C 


INDEX  OF  SUBJECT  MATTEK 


573 


Factors  influencing  rise  of  soil  tempera- 
ture,  231. 
Farm   manure,   a   direct  and  indirect  fer- 
tilizer,  504. 
agricultural  value  of,  513. 

agricultural    value   of   protected   ma- 
nure,  525. 

amounts  applied,   527. 

amount  produced  by  cows,  514. 

amounts  produced   by   farm    animals, 
513. 

amount  produced  by  horses,  514. 

amount  produced  by  poultry,   514. 

amount  produced  by   sheep,    514. 

amount  produced  by  steers,   514. 

amount  produced  by   swine,    514. 

average  composition  of,   504. 

care  of  in  stalls,   520. 

characteristics   of,    500. 

commercial    evaluation   of,    512. 

composition  of  from  various  animals, 
501. 

covered  yards  for,  523. 

effect  on  carbon  dioxide  of  soil  air, 
254. 

efficient  application  of,   525. 

evaluation  of,  512. 

factors    influencing    composition    of, 
506. 

fermentation     and     putrefaction     of, 
508. 

fresh  and  well  rotted  compared,  511. 

fresh  and  yard,  crop  effects,  512. 

hauling  directly  to  field,   521. 

importance  of,  499. 

importance  of  its  decay,  511. 

importance  of  protection,   524. 

importance  of  tight  floors,   521. 

influence  of  handling  on,  507. 

influence  of  tramping  on,  524. 

influence  on  cotton,   535. 

influence  on  maize,  534. 

influence  on  meadows,  532. 

influence  on  potatoes,  534. 

influence  on  tobacco,  535. 

liquid   and  sr1^'   -"~>rared,   502. 

loss  of  const  >u,    508,   515. 

losses    during  and    storage, 

519. 

maintenance    of    toll    organic    matter 
by,  124,  519.  559. 

modern  mai iu  ial    i>\       ice,  520. 


Farm  manure,  nutrient  losses  during  pro- 
duction,  516. 

outstanding  characteristics  of,   505. 

piles  outside,   522. 

pits  for,  523. 

place  in  rotation,  532. 

produced  by  animals,   calculation  of, 
514. 

products  of  decay,   510. 

reinforcement  of,    527. 

residual  effects  of,   531. 

resume  of  use,   535. 

use  of  in  sulfur  composts,  406. 

use  of  lime  with,  530. 

use  of  litter  with,  521. 

use  in  composting,   530. 

variability  of,  506. 
Feldspar  as  a  fertilizer,   465. 
Fermentation,  defined,   103,  410. 

of  farm  manure,   508. 
Fertility,  maintenance  of  as  influenced  by 

different  types  of  farming,  558. 
Fertility  of  soil,   defined,  554. 

effect  on  transpiration  ratio,   192. 

possible  exhaustion  of,  308. 
Fertility     evaluations     by     means     of     a 

chemical  analysis,   323. 
Fertilization,    systems  of,    496. 
Fertilizers,    advantages    of    home    mixing, 
485. 

amounts  to  apply,  492. 

brands  of,  478. 

calculations  of  for  home-mixing,  487. 

carrying  free  sulfur,   467. 

catalytic,    466. 

containing  nitrogen,   444. 

containing  phosphorus,  454. 

containing  potash,   463. 

development  of  their  use.  442. 

early  use  of,   442. 

effect  of  on  soil  acidity,  353. 

element  in  the  minimum,   476. 

factors    which    determine    the    choice 
of,   488. 

farm  manure,    504. 

formulae,  for  different  soils  and  crops, 
491. 

formulas,   nature  of,   489. 

formulas,  theory  of,  490. 

function  of,  444. 

guarantees  of,   481. 

how  to  buy,    483. 


574 


INDEX  OF  SUBJECT  MATTER 


Fertilizer,  how  to  home  mix,  487. 

importance  of  high  grade,   483. 

importance  of  residues  from,   295. 

inspection  and  control,   480. 

interpretation  of  guarantee,    481. 

laws  of,  480. 

law  of  diminishing  returns,  493. 

low  grade  and  high  grade,  479. 

method  and  time  of  application  of, 
495. 

purchase  of  unmixed,  484. 

rational  utilization  of,   497. 

systems  of  applying,   496. 

use  in  United  States,  444. 

which  should  not  be  mixed,   486. 
Fertilizer  mixtures,  those  of  value,   487. 
Fertilizer   practice,    principles  of,    471. 

rational  system,    497. 
Fertilizer  residues,   cause  of,   294. 

nature  of  from  different  salts,  294. 
Fillers,  use  of  in  fertilizers,  488. 
Fineness  of  limestone,  data  as  to  impor- 
tance,  377. 

importance  of  in  liming,  377. 

influence  of  on  decomposition,  378. 
Fish  scrap,   composition  of,    446. 
Fixation  of  nitrogen  artificially,   450. 

Bucher  method,  453. 

carbide  method,  451. 

electric  arc  method,  452. 

Haber  method,   453. 
Fixation   of    nitrogen   by   free-living   soil 
organisms,  430. 

by  nodule  bacteria,   433. 
Floats,  see  rock  phosphate,  455. 
Flocculation,  cause  of,   131. 

defined,  130. 

relation  of  to  granulation,   144. 
Flood  plain  soils,  47. 
Floors,    importance    in    care    of    manure, 

521. 
Flue  dust,  a  source  of  potash,  465. 
Food  for  plants,  defined,  8. 
Forms  of  water  in  soil,  diagram  of,  199. 
Forms  of  soil  water,  151. 
Forms  of  lime  to  apply,  367. 
Formulae  of   fertilizers  for  different  soils 
and  crops,  491. 

examples  of,   489. 

theory  of,  490. 
Freezing    and    thawing,    effect    on    soils, 
23. 


Frost,  importance  in  soil  formation,  23. 
Fungi  and  algae,  smaller  forms  in  soil, 
388. 

fixation  of  nitrogen  by,  432. 

injurious  to  higher  plants,   396. 

in  soil,  number  of,  388. 

large  forms  in  soil,  386. 

Germination    of    seeds,     temperature    of, 

224. 
Geological  classification  of  soils,  38. 

resume  of,   65. 
Glacial  lakes,  origin  of,  59. 

soils  of,  58. 
Glacial    soils,    chemical    composition    of, 
57. 

compared  with  residual,  57,  58. 

fertility  of,   57. 

general  character,  54. 

importance  of,  57. 

origin,  54. 
Glaciation,  American  ice  sheet,   53. 

effect  of,   54. 

influence  on  agriculture,  58. 

in  North  America,  54. 
Glaciers  in  soil   formation,   18. 
Grading  of  ground  limestone,  377. 
Grandeau  method,  nature  of,  312. 
Granite,  chemical  composition  of,   33. 

weathering  of,  33. 
Grass,   influence  on  nitrate  accumulation, 
427. 

influence  on   nitrification,    422. 
Granulation     of    soil,     as     influenced    by 
lime,    143. 

beneficial   effects,    141. 

defined,  139,  141. 

forces  producing,  143. 

influence  of  plowing  on,   146. 

influence  of  tillage  on,  144. 

production  of,  142. 
Gravity    water,    amount    soil    will    hold, 
177. 

calculation  of,   178. 

factors  affecting  movement,  175. 

importance  of  study,  178. 
Green  manures,  ancient  use  of,   537. 

as  cover  crops,   538,   541. 

constituents  gained  by  use  of,  539. 

crops  for,  545. 

decay  of  in  soil,  541. 

general  influence  of,   538. 


INDEX  OF  SUBJECT  MATTER 


575 


Green  manures,  importance  of,  537. 

influence  of  decay  of,  543. 

influence  of  decay  on  lime  and  phos- 
phorus,  545. 

influence   of    decay   on   nitrate   accu- 
mulation,  544. 

influence  on  crops,   552. 

influence  on  nitrate  reduction,  426. 

manner  of  turning  under,   549. 

practical  utilization  of,  552. 

relation  of  to  the  rotation,  552. 

relation  to  humus  formation,   543. 

relative  value  of  different  crops  for, 
547. 

time  for  plowing  under,  548. 

to  maintain  organic  matter,   123. 

use  of,   548. 

use  of  lime  with,  550. 
Ground  limestone,    365. 
Guano,  nature  of,  445. 
Guarantees    on    fertilizers,    statement    of, 

481. 
Gullying  and  its  control,  206. 
Gypsum  as  a  soil  amendment,  379. 

effect  of  on  soils,  379. 

reinforcement  of  manure  with,  527 

use  of  in  alkali  control,  342. 

Haber -method  of  fixing  nitrogen,  453. 
Handling  of  manure,   covered  yards,   523. 

hauling  directly  to  field,   521. 

influence  on  composition,  507. 

manure  pits,  523. 

piles  outside,   522. 

care  of  in  stalls,  520. 
Hematite,  as  a  soil  color,  37. 

change  of  to  limonite,  27. 

formation  of  in  soil,   25. 

source  of  in  soil,  7. 
Heat,   conduction   of,    240. 

conduction  of  in  soil,  234. 

conductivity  of  various  soils  for,  235. 

convection   transfer   of,    238. 

cycle   between    soil    and    atmosphere, 
225. 

factors  affecting  conduction  of,   235, 


evaporation  loss  by,  241. 
importance   in   soil   formation, 
loss  of  from  soil,   240. 
movement  in  soil,   234. 
radiation  of,  240. 


21. 


Heat  of  wetting  of  soils,  amount  of,  153. 
data  of,    154. 
effect  of  texture  on,    154. 
significance  of,  153. 
High    grade    fertilizers,     importance    of, 

4£3. 
Higher   plants,    influence   on   nitrification, 

422. 
Home-mixing    of     fertilizers,     calculation 
of,   487. 
advantages,  485. 
good  mixtures  for,  487. 
how  performed,  486. 
Hoof  meal,  composition  of,  446. 
Humid  soil,  biological  activity  in,  32. 
chemical  analysis  of,  31. 
humus  content  of,   120. 
Humidity,     influence    of    on    the    hygro- 
scopic coefficient,   158. 
Humus,  amount  in  California  soils,  120. 
amount  in   Nebraska  loess,   120. 
defined,  115. 

determination  of,   115,  312. 
formation    of    from    green    manures, 
543. 
Hydration,  influence  of  in  soil  formation, 

26. 
Hydrogen-ion     concentration,     a     measure 
of  soil  acidity,  356. 
method  of  expression,   350. 
relation  to  soil  acidity,   346. 
Hydrolysis,   explanation  of,   348. 

production  of  by  enzymes,   390. 
Hygroscopic    capacity   of    soils,    data    on, 

156. 
Hygroscopic    coefficient,    data    as    to    spe- 
cific soils,  157. 
defined,   152. 
determination  of,  154. 
factors  affecting,  157. 
range  of  in  soils,   158. 
Hygroscopic  water  of  soils,  specific  char- 
acter of,   153. 

Ice,   disintegration  of  rocks  by,   23. 
glacial,   in  soil  formation,  18. 

Ice  action,  glaciation,   53. 

Ice  age,  53. 

Ignition  method  for  determining  soil  or- 
ganic matter,  112. 

Influence    of    alkali,    condition    affecting, 
338. 


576 


INDEX  OF  SUBJECT  MATTER 


Inoculation    of    soil    with    B.    Radicicola, 

methods  of,  439. 
Insulation,    absorbed    by    earth's    atmos- 
phere, 226. 
absorbed  by  soil,  227. 
absorption  of  as  influenced  by  color, 

228. 
absorption  of  as  influenced  by  slope, 

229. 
received  by  soil,  225. 
Insoluble  phosphoric   acid,    defined,    456. 
Inspection  and  control  of  fertilizers,  480. 
Ions,   absorption  of  by  soils,   270. 
differential   diffusion   of,    293. 
diffusion  of  into  plants,   291. 
Ionization,   defined,   270. 

of  water,  270. 
Irrigation,  relation  to  rise  of  alkali,  339. 
Irrigation   water,   alkali   content  of,    333. 
Iron,  in  soil  minerals,  7. 

relation    to    the    reversion    of    acid 

phosphate,  457. 
relation  of  to  soil  acidity,  347. 

Kainit,    reinforcement    of    manure    with, 

527. 
Kaolinite,  importance  in  soils,  7. 

source  of  in  soil,  6. 
Kelp,  a  source  of  potash,  465. 
Kjeldahl    method    for    determination     of 

nitrogen,  312. 

Lacustrine  soils,  character  of,   60. 

glacial  lake,  58. 

importance  of,   60. 

location  in  U.  S.,  60. 

recent  lake,  60. 
Lake  salines,  a  source  of  potash,  465. 
Law  of  diminishing  return,  493. 
Leaching,  effect  of  on  soil  acidity,  352 

loss  of  lime  thereby,  370. 

use  of  in  alkali  eradication,  341. 
Leather  meal,  composition  of,   446. 
Legumes,  inoculation  of,   438. 
Leguminous    crops,    amounts    of    nitrogen 
fixed  by,  438. 

cross  inoculation  of,  434. 

effect  on  soil  nitrogen,  437. 

nitrogen  fixation  by,  433. 
Leucite  as  a  fertilizer,  465. 
Lime,  agricultural  terminology  of,  364. 

agricultural  use  of,  363. 


Lime,  amounts  to  apply,  368. 

biological  effects  in  soil,  371. 
burned,  364. 
carbonated,    365. 
cause  of  crop  response  to,   372. 
changes  in  soil,   369. 
chemical  effects  of  on  soil,  871. 
composition  of  as  sold  in  Pennsylva- 
nia,  366. 
contact  action  of,  374. 
conversion  factors  of,  367. 
crop   response  to,   372. 
effect   of    caustic    forms    on    manure, 

530. 
forms  of,  363. 
forms  to  apply,  367. 
importance   of   in   soil    improvement, 

381. 
influence  of  green  manures  on,  545. 
influence  on  availability  of  nutrients, 

373. 
influence  on  decay  of  green  manures, 

551. 
influence  on  granulation,   143. 
influence  on  nitrification,  373. 
influence  on  soil  bacteria,   395. 
influence  on  sulfofication,  405. 
losses  from  Cornell  soils,  307,  370. 
methods  of  applying,   374. 
need  of  determinations,   365. 
physical  effects  on  soil,   371. 
problem  showing  form  to  buy,   368. 
proper  utilization  of,   382. 
relation     of    to     fertilizer    mixtures, 

486. 
relation  to  reversion  of  monocalcium 

phosphate  in  soil,  457. 
relation   to   the   use   of   manure   and 

fertilizers,  382. 
time  to  apply,  374. 
use  of  manure  with,  530. 
use  of  with  green  manure,   551. 
water  slaked,    364. 
Lime      requirement      determinations,      on 

soils,  355. 
types  of,  355. 
Lime  requirement  of  soils,  Veitch  method, 

356. 
Limestone,  amounts  to     ;>ply,  368. 
burning  o 

changes  ii    k    .  370. 
chemical  of,  33. 


INDEX  OF  SUBJECT  MATTER 


577 


Limestone,    conversion   factors,    367. 

fineness  of  average  product,  378. 

fineness  of  for  agricultural  use,   367. 

grading  of  as  to  fineness,   377. 

importance  of  fineness,   377. 

mechanical    composition    as    sold    in 
Pennsylvania,    379. 

ratio  of  calcium  and  magnesium,  375. 
Liming,  amounts  of  lime  to  apply,  368. 

calcium     and     magnesium     ratio     of, 
375. 

cause  of  crop   response  to,    372. 

crop  response  to,   372. 

forms  of  lime  to  apply,  367. 

importance   of   in   soil   improvement, 
381. 

method   and   time   of   applying  lime, 
374. 

reasons  for,  362. 
Limonite,  source  of  in  soil,  7. 

production  of  from  hematite,   27. 

weathering  of,   33. 
Limonite  group,  as  soil  color,  37. 
Linseed  meal,   composition  of,   446. 
Lithosphere,    composition   of   compared   to 

soils,   13. 
Litmus  paper  test,  criticism  of,  360. 

procedure,    358. 

use  of  potassium  nitrate  with,  358. 
Litter,  absorptive  power  of,  521. 

influence    on    character    of    manure, 
521. 
Loam,   defined,   82. 
Loess,  a  wind  laid  soil,  21. 

character  of,   62. 

chemical  composition  of,  63. 

importance  of,  63. 

location  of,   62. 

minerals  of,  62.  ^ 

origin  of,  61. 
Loss  of  nutrients  from  soil,  554. 

types  of,  289. 
Loss  of  soil  heat,  by  conduction,  240. 

by  evaporation,   240. 

by  radiation,  240. 
Loss  of  soil  water  by  run  off,  203. 
Losses    during    the  and    han- 

dling of  n 
Losses   from   and   a  soils   under 

various  ty;  ing,  558. 

Lysimeter  experime: '■.        Bi     nberg,  272. 
306. 


Lysimeter    experiments,    at    Cornell    Uni- 
versity,   307. 
at     Rothamsted     Experiment     Farm, 
180,  217,  288. 
Lysimeters,  nature  of,   180. 

of  Cornell  University,    181. 
of    Rothamsted    Experiment    Station, 
180. 

Macro-organisms  of  the  soil,  384. 
Maintenance  of  soil  fertility,   554. 

influence  of  different  types  of  farm- 
ing on,  558. 

program  of,  560. 
Maintenance  of  soil  organic  matter,   122. 
Maize,  influence  of  farm  manure  on,  534. 

influence     on     nitrate     accumulation, 
428. 

influence  on  nitrification,  422. 
Manganese,  relation  of  to  soil  acidity,  347. 
Mangum  terrace,  205. 
Manurial  practices,   phases  of,   520. 
Marine  soils,  character  of,  51. 

chemical   composition  of,    52. 

importance  of,  51. 

origin  of,   50. 
Marl,  origin  and  nature  of,   45. 

term  defined,  45. 

use  of,  45. 
Maximum    retentive    power    of    soil    for 
water,  162. 

determination  of,  162. 
Maximum    water    capacity    of    soils,    data 

on,   166. 
Meadows,  influence  of  manure  on,  532. 
Mechanical  analysis  of  soils,   67. 

beaker  method,   69. 

Bureau  of  Soils  method,  71. 

determination  of  soil  class  from,   84. 

value  of,  79. 
Mechanical  analyses  of  typical  soils,  83. 

of  various  soils,  81. 
Methods  of  applying  fertilizers,  495. 
Methods  of  studying  drainage  losses,   180. 
Micro-organisms   of   the   soil,    386. 
Micron,   magnitude  of,    128. 
Millimicron,  magnitude  of,   128. 
Mineral  constituents  of  soils,  bulk  analy- 
sis of,  314. 

extraction  of  with  dilute  acids,  317. 

extraction  of  with  strong  acids,  316. 

extraction  of  with  water,  319. 


; 


578 


INDEX  OF  SUBJECT  MATTER 


Mineral    cycles    in    soils,    importance    of, 
408. 

nature  of,  407. 

organisms  of,  408. 

types  of,  407. 
Minerals  in  earth's  crust,  4. 
Minerals  of  the  soil,  77. 

importance  of,  6. 

list  of,  5. 

source  of,  4. 

specific  gravity  of,  89. 
Minerological  analysis  of  soils,  76. 
Minerological  character  of  soils,   77. 

of  soil  particles,  75. 
Minimum,  element  in  the,  476. 

law  of  Mitscherlich,   477. 
Modification  of  soil  air,  261. 
Moisture  of  soil,    conservation   by  mulch, 
221. 

conservation,  weed  control,   220. 

control,  summary  of,  221. 

data     for     sandy    and     clayey     soils, 
179,  200. 

determination    on    soil,     method    of, 
161. 

effect  on  heat  conductivity,  236. 

effect  on  movement  of  soil  air,  258. 

effect  on  nitrification,  420. 

effect  on  specific  heat  of  soils,  233. 

influence  on   nitrification,    420. 

influence  on  bacteria,   393. 
Moisture  equivalent  of  soils,  defined,  167. 

for  various  soils,  168. 

method  of  determination,    167. 
Molecules,  absorption  of  by  soil,  269. 
Moraines,   agricultural  value,   54. 

ground,   54. 

terminal,   54. 
Movement    of   soil   air,    factors   affecting, 

258. 
Muck,   agricultural  value  of,   43. 

capacity  for  water,  164. 

character  of,   43. 

chemical  analysis  of,  44. 

term  defined,  43. 
Mulch,  artificial,   218. 

soil,  use  of,  218. 
Muscovite,   change  of  to   kaolinite,    26. 

present  in  soils,  5,  77. 

Nitrates  in  alkali  spots,  332. 
Nitrates  in  rain  water,  data,   429. 


Nitrates  in  soils,  accumulation,  419. 

accumulation   as   influenced   by  green 

manure,  544,  549. 
accumulation,    influence   of   grass   on, 

427. 
accumulation,   influence  of  maize  on, 

428. 
as   a   source   of    nitrogen    for    higher 

plants,  415. 
assimilation    of    by    soil    organisms, 

426,  428. 
influence  of  green  manures  on,  543. 
production  of,  111. 
reduction  of,  424. 
Nitrate  reduction,  cause  of,  425. 
control  of,  426. 

influence   of  green  manures  on,    426. 
influence  of  straw  on,  425. 
nature  of,   425. 
organisms  of,  425. 
Nitrification    in    soil,   as   affected   by   soil 

conditions,  418. 
as  influenced  by  carbon  dioxide,  256. 
effect  of  aeration  on,  418. 
effect  of  alkali  on,  421. 
effect  of  carbon  dioxide  on,  419. 
effect  of  farm  manure  on,  418. 
effect  of  moisture  on,   420. 
effect  of  soil  acidity  on,  421. 
effect  of  temperature  on,   420. 
efficiency  of,  417. 

influence  of  higher  plants  on,  422. 
influence  of  lime  on,  373. 
influence  of  previous  crops  on,  423. 
influence   on   carbon   dioxide   produc- 
tion, 255. 
nature  of,  415. 
organisms  of,  415. 
products  of,  415. 
reactions,  415. 

relation  to  ammonifi cation,   416. 
relation  of  to  carbon  cycle,  408. 
relation  of  to  mineral  cycle,  408. 
relation  to  soil  fertility,  423. 
Nitrifying  organisms,   types  of,    415. 
Nitrites,  production  of  in  soils,   1  1. 
Nitrobacter  in  soil,  415. 
Nitrogen,  additions  to  soil,  by  free-fixing 

organisms,   430. 
additions  to  soil,  in  manure,  557. 
additions  to  soil,  in  rain  water,  429. 
additions  to  soil,  modes  of,  429. 


INDEX  OF  SUBJECT  MATTER 


579 


Nitrogen,  additions  to  soil,  nature  of,  429. 
amount  fixed  by  B.   Radicicola,  437. 
amount  in  ammonium  sulfate,   449. 
amount  in  calcium  cyanamid,  452. 
amount  in  calcium  nitrate,  452. 
amount  in  California   soils,   120. 
amount  in  dried  blood,   445. 
amount   in  Nebraska   loess,   120. 
amount  in  sodium  nitrate,  448. 
amount  in  soils,   12. 
amount  in  soils  of  United  States,  118. 
amount  in  tankage,   445. 
artificial   fixation   of,    450. 
availability  of  in  fertilizers,  454. 
contained  in  rocks,  10. 
determination  of  in  soils,   312. 
fixation  by   B.   Radicicola,   433. 
fixed  in  soil  by  azofication,  433. 
forms  of   in  soil,    10. 
from    B.    Radicicola,   availability    of, 

437. 
gain  due  to  green  manures,  539. 
gain  due  to  natural  causes,  429. 
importance    in    biological    processes, 

409. 
importance  of  in  fertility  evaluation, 

323. 
importance  of  in  soils,  409. 
in  farm  manure,   501. 
in  liquid  and  solid  manure,  503. 
in  rain  water,   data   on,   429. 
inert  character  of,   409. 
influence  on  plant  growth,   471. 
losses      from      Bromberg     lysimeters, 

306. 
losses   from   Cornell   soils,    307. 
losses  from  decaying  manure,   511. 
losses   from   farm   manure,    519. 
losses  from  soil,  555. 
natural  addition  to  soil,   557. 
of    food    recovered    in   farm    manure, 

516. 
organic  forms  used  by  plants,  411. 
possible     detrimental     influences     of, 

473. 
x  relation  of  to  life,  409. 
removed  by  crop?  from  Cornell  soils, 

325. 
utilization      of      organic     forms     by 

plants,  447. 
Nitrogen    cycle,    ad  of    nitrogen    to 

soil  by  free-fixing  bacteria,   430. 


Nitrogen    cycle,    addition    of    nitrogen    to 
soil  in  rain  water,   429. 
ammonification,  412. 
assimilation    of    nitrates    by    soil    or- 
ganisms, 426. 
complexity  of,  409. 
decay  and  putrefaction,   410. 
fixation    of   nitrogen   by  Azotobacter, 

432. 
fixation    of    nitrogen    by    B.    Radici- 
cola, 433. 
nitrification,    415. 
reduction  of  nitrates,   424. 
relation  of  to   other   cycles,    410. 
Nitrogenous     fertilizers,     ammonium     sul- 
fate, 449. 
calcium   cyanimid,   451. 
calcium  nitrate,  452. 
castor  pomace,   446. 
cotton  seed  meal,  446. 
dried  blood,  445. 
fish  scrap,  446. 
guano,   446. 
hoof  meal,   446. 
leather  meal,  446. 
linseed  meal,  446. 
process  goods,   446. 
relative  availability,  454. 
sodium  nitrate,  448. 
tankage,  445. 

utilized  by  higher  plants,  411,  447. 
wool  and  hair  waste,    446. 
Nitrosomonas  in  soils,   415. 
Nitrous    acid,    relation    to   mineral    cycle, 

408. 
Nodules  on  the  roots  of  leguminous  plants, 

nature  of,   434. 
Number  of  particles  in  soil,   96. 
Number  of   soil   particles,    calculation   of, 

96. 
Nutrient      elements      used      by      plants, 
amounts  in  soil,    12. 
defined  and  explained,  8. 
listed,   9. 
primary,  10. 
source  of,   10. 
Nutrients  in  soils,  addition  by  leguminous 
green  manures,   557. 
addition  of  in  farm  manure,   557. 
differential      diffusion      into      plants, 

293. 
diffusion  into  plants,   291. 


580 


INDEX  OF  SUBJECT  MATTER 


Nutrients    in    soils,     direct    influence    of 

plants  on  solubility  of,   295. 
how  lost  from  the  soil,   289. 
influence    of    lime    on    solubility    of, 

373. 
lost  by  drainage  and  cropping,  307. 
lost  by  leaching,    Cornell   data,   210. 
lost  by  plant  influence,   303. 
lost     during      manurial      production, 

cows,   516. 
lost     during     manurial     production, 

heifers,   516. 
lost      during      manurial      production, 

sheep,   516. 
lost     during     manurial     production, 

steers,   516. 
lost  from  Cornell  soil,  554. 
lost  from  soil,  relative  losses,  308. 
lost  in  handling  and  storage  of  ma- 
nure,   517. 
natural  additions  of  to  soil,  556. 
quantities  removed  by  crops,  data  of, 

303. 
recovery  of   in  farm   manure,   516. 
solubility    as    influenced    by    carbon 

dioxide,   255. 
Nutrient  losses  from  and  addition  to  soil 

under  various  types  of  farming, 

558. 

Ocean,  soils  found  in,  50. 

Ohio    results    with    raw    rock    phosphate, 

462. 
Optimum     soil     moisture,     influence     of 
structure  on,   201. 
for  plant  growth,  200. 
Organic  carbon,   determination  of,   311. 

use  of  by  higher  plants,   402. 
Organic  compounds  of  soil,   character  of, 
105. 
classification  of,  107. 
nitrogenous,    106. 
relation  to  plants,  108. 
Organic    decay,    effect    on    soil    tempera- 
ture, 239. 
Organic    decomposition,    simple    products 

of,  110. 
Organic     matter,     amount     in     Nebraska 
loess,  120. 
amount  in  soil  of  United  States,  117. 
decay  of,  103. 
compounds  isolated  from,  108. 


Organic  matter,   defined,   99. 

determination    of   in   soils,    112. 

effect  of  on  soil  acidity,  353. 

effect   on   capillary   capacity   of   soil, 

164. 
effect  on  carbon  dioxide  of  soil   air, 

253. 
effect  on  specific  heat,  233. 
influence  in  soil,  8. 
influence  of  soil  conditions  on  decay 

of,   124. 
influence     on     availability     of     rock 

phosphate,  460. 
influence  on  the  soil,   121. 
in  Minnesota  soils,   119. 
maintenance  of  in  soil,   122. 
Organic    matter    of    soil,    effect    on    heat 
conductivity,    236. 
general  nature,  7. 
influence  on  bacteria,   394. 
portion  alive,  100. 
sources  of,  7,  99. 
Organic    nitrogenous    compounds,    utiliza- 
tion by  higher  plants,  411,  446. 
Organic    nitrogenous    fertilizers    of    secon- 
dary importance,   445. 
Organic  toxins,   elimination  of,    109. 

of  soil,   108. 
Organisms,  benefits  of  in  soil,  397. 

pounds  of  in  soil,  384. 
Orthoclase,  change  of  to  koalinite,  26. 

importance  in  soil,  6. 
Osmosis,   defined,   289. 

how  demonstrated,  290. 
pressure  developed  by,    290. 
Osmosis   of  water  into  plants,   290. 
Osmotic  pressure,   nature  of,  290. 
Oswald    method    of    converting    ammonia 

into   nitric  acid,   453. 
Outlets    for   tile   drains,    construction    of, 

214. 
Oxidation,  effect  of  on  composition  of  soil 
air,  254. 
importance  of  in  soil   formation,    24. 
of  sulfur   in   soil,   403. 
Oxidases,    production    of    by    plant    roots, 

297. 
Oxygen,   importance  in  soil  air,   256. 

Packers,  action  of,   148. 
Partial  analysis  of  soils,   315. 

digestion  with  dilute  acids,  317. 


INDEX  OF  SUBJECT  MATTER 


581 


Partial    analysis    of    soils,    digestion    with 
dilute  acids,  objections,  318. 

digestion    with    dilute    acids,    value, 
318. 

digestion  with  strong  acids,    316. 

digestion    with    strong    acids,    objec- 
tions, 316. 

extraction  with  water,  319. 

extraction    with    water,    method    of, 
321. 

extraction  with  water,   value,   322. 

of  Minnesota  soils,  316. 

of     Minnesota     and     Maryland     soils, 
317. 
Partially  decomposed  matter  of  soils,  105. 
Particles  of  soil,  number  to  a  gram,  96. 
Peat,  agricultural  value  of,  43. 

capillary  capacity  of,   164. 

character  of,  43. 

chemical   analysis   of,    44. 

term  defined,  43. 
Percolation,    control   of,    208. 

Cornell  data,  209. 

effects  of  crops  on,  210. 

loss  of  nutrients  by,   208. 

in  arid  regions,  208. 

in  humid  regions,  208. 

Rothamsted  data,   207. 
Ph  values  of  acidity  explained,   350. 
Phosphate     fertilizers,      acid     phosphate, 
456. 

basic  slag,  457. 

bone  phosphate,   454. 

relative   availability   of,    458. 

rock  phosphate,   455. 
Phosphoric  acid,  amount  of  in  acid  phos- 
phate, 456. 

amount  of  in  apatite,   6. 

amount  of  in  basic  slag,  457. 

amount   of   in   igneous   rocks,    6. 

amount  of  in  manure,   501. 

amount  of  in  bone,  454. 

amount  of  in  rock  phosphate,  455. 

amount  of   in  soils,    12. 

forms   of  in  fertilizers,    456. 

forms  of  in  soil,   11. 

influence   of   green   manures  on,   545. 

influence    of    lime    on    reversion    of, 
373. 

influence  on  plant  growth,   474. 

in  liquid  and  solid  manure,  503. 

loss  of  from  farm  manure,   519. 


Phosphoric  acid,  loss  of  from  soil,  555. 
losses  from  Cornell  soils,   307. 
of    food    recovered    in    farm    manure, 

516. 
of  soil,   140. 

organic   and   inorganic  in  soils,   315. 
organic  nature  of,    11,   314. 
pounds    removed    by    various    crops, 

468. 
relative  availability  of  in   fertilizers, 

458. 
Phosphorus,  see  phosphoric  acid. 
Physical   absorption  by  soils,    263. 
Physiological    character    of    plants    in   re- 
lation to  alkali  toxicity,  336. 
Piconometer,  for  determination  of  specific 

gravity,  90. 
Plankers,    action    of,    148. 
Plants,    absorptive    activity   of    as    deter- 
mined by  certain  factors,  299. 
acquisition   of   nutrients  by,   291. 
alkali  vegetation,   340. 
capacity   of   to   grow   on   poor   soils, 

299. 
cause  of  drought  resistance  by,    195. 
cause  of  wilting,  194. 
detrimental  influence  of  nitrogen  on, 

473. 
differential    diffusion    into,    292. 
different  absorptive  capacity  for  soil 

nutrients,     301. 
direct     influence     of    upon    soil     nu- 

nutrients,    296. 
effect  of   alkali   on,    334. 
effect     of     calcium     and     magnesium 

ratio   on,   376. 
effects    of    on    percolation,    208. 
factors    affecting    transpiration    from, 

188. 
function    of    water    in,     184. 
growth   of   in  acid  medium,   350. 
influence    of    on    the    soil    solution, 

284. 
influence   of   phosphorus   on,    474. 
influence   of   potassium   on,    475. 
influence    of    roots    on    soil    colloids, 

297. 
influence   of    soil    water   on,    186. 
production    of    acids    by,     296. 
productions    of    oxidases   by,    297. 
reduction  produced  by  roots  of,   297. 
resistance    of    to    alkali,    335. 


582 


INDEX  OF  SUBJECT  MATTER 


Plants,   response  of  to  lime,  372. 

soil  organisms  injurious  to,   396. 

tolerance  of  to  soil  acidity,   353. 

used  for  green  manures,    546. 

utilization  of  ammonia  by,  415,  450. 

utilization  of  organic  carbon  by,  402. 

utilization    of    organic    nitrogen    by, 
411,    447. 

water    requirements    of,    187. 
Plants     and     animals,     relation     to     soil 
%  formation,   23. 

Plant  diseases,  control  of  in  soil,   397. 
Plant    food,     denned,     8. 
Plant    growth,    factors    for,    8. 

influence    of    nitrogen    on,    472. 

optimum    moisture    for,    200. 

temperature    for,    224. 
Plant  nutrients,  amounts  in  soil,  12. 

contained    in    minerals,     5. 

defined,    8. 

derived    from    air,    9. 

derived    from    soil,    9. 

listed,   9. 

primary,    10. 
Plant  roots,  production  of  carbon  dioxide 
by,   252. 

prying   effect    on    rocks,    23. 
Plant    tissue,    composition    of,    100. 

method    of    analysis,    102. 
Plasmolysis,    defined,    290. 
Plowing,  influence  of  on  the  soil,  146. 
Pore   space   of   soils,    calculation   of,    94, 
178. 

data    on,    95. 

importance    of,    95. 

nature    of,     93. 
Potash,  amount  in  soils,  12. 

fertilizers     carrying,     463. 

forms  of  in  soil,  11. 

in  farm   manure,    501. 

influence  on  plant  growth,   475. 

in  liquid  and  solid  manure,   503. 

in    minerals,    6. 

loss    of    from    soil,    555. 

loss  of   from   farm  manure,    519. 

losses  from  Cornell  soils,  307. 

miscellaneous    fertilizers    of,    464. 

of    food    recovered    in    farm    manure, 
516. 
Potash   fertilizers,   alunite,    465. 

feldspar,   465. 

flue   dust,    465. 


Potash  fertilizer,  kelp,  465. 

lake  salines,    465. 

leucite,     465. 

Stassfurt    salts,     463. 

wood    ashes,    464. 
Potassium,  see  potash. 
Potassium  chloride  as  a  fertilizer,  463. 
Potassium  nitrate,  use  of  in  litmus  test, 

358. 
Potassium  sulfate  as  a  fertilizer,   464. 
Poultry   manure,    character   and   composi- 
tion   of,    503. 
Potatoes,   influence  of  manure  on,   534. 
Practical    soil    management,    factors    of, 

560. 
Precipitation,  addition  of  sulfur  by,  469. 
Pressure,  effect  on  gravity  water,  175. 

effect  on  movement  of  soil  air,  260. 
Process    fertilizers,    nature    of,    446. 
Productivity,   as   influenced   by   soil   solu- 
tion,  287. 

equation  of,   327. 
Protection,   influence  of  on  farm  manure, 

525. 
Proteid    compounds,    changes    of    in    de- 
caying manure,  510. 
Protozoa,    importance   of    in    soil,    387. 

number  of  in  soil,  387. 

relation    of    to    ammonification,    387. 

types    of    in    soil,     387. 
Puddling   of    soils,    141. 
Purchase  of  commercial  fertilizers,  483. 
Purchase  of  unmixed  fertilizers,   484. 
Putrefaction,    defined,    103,    410. 

of    farm    manure,     508. 

products    of,     411. 

Qualitative      composition      of      drainage 
water,  304. 
of    the    soil    solution,    280. 
Quantitative      composition      of      drainage 
water,  304. 
of    soil    solution,    282. 
Qualitative  tests  for  soil  acidity,  358. 

compared    and    criticized,     359. 
Quantitative  tests  for  soil  acidity,   nature 
of,  355. 
value    of,    357. 

Radiation,  loss  of  heat  from  soil  by,  240. 
Rain-water,   analysis  of,    429. 
sulfur  in,  404. 


INDEX  OF  SUBJECT  MATTER 


583 


Rational    fertilizer  practice,    497. 
Recovery    of    nutrients    in    farm    manure, 

516. 
Reduction,    as    affected    by    plant    roots, 

297. 
Reduction   of    nitrates    in    soil,    424. 
Reinforcement  of  farm  manure,   527. 
agricultural     value    of,     529. 
balancing    influence,     529. 
conserving     effects,     529. 
Residual  influence  of  manure,  531. 
Residual  soils,   age  of,   39. 
analysis    of,     33,     41. 
chemical    composition    of,    52,    57. 
compared  with  glacial  soils,   57. 
colors    of,    32,    39. 
formation  of,   38. 
from    specific    rocks,    39. 
location  of  in  U.  S.,  41. 
organic     content,     41. 
Residues    in    soil    from    differential    dif- 
fusion, 294. 
Resistance  of  plants  to  alkali,  generalized 

table  of,  338. 
Resistance    to    alkali    by    various    plants, 

data  on,   338. 
Reversion    of    mono-calcium    phosphate    in 

soil,   457. 
Reverted  phosphoric  acid,  defined,  456. 
Rock   phosphate,    as    a    reinforcement    for 
manure,    528. 
changes    in    soil,    456. 
compared   with   acid   phosphate,   458. 
composition,     456. 
composted    with    manure,     462. 
influence  of  organic  matter  on,  460. 
Ohio    results   on,    462. 
source,    455. 

use  of  in  sulfur  composts,    406. 
Rocks,    igneous,    sedimentary    and    meta- 
morphic,   4. 
soil    forming,     3. 
Rodents,    macro,    soil   organisms,   384. 
Rollers,    actions    of,    148. 
Roots  of  higher  plants,  a  type  of  macro- 
organism,   386. 
production     of     carbon     dioxide     by, 

295. 
production   of   exudates   by,    296. 
Rooting    habit    of    plants,    in    relation   to 

alkali   toxicity,   336. 
Rotation,  farm  manure  and  the,  532. 


Sampling    of    soil,    method    of,    311. 
Sand  dunes,  nature  of,  64. 
Season,  influence  on  soil  solution,  283. 
Sedentary  soil,  explanation  of  term,  28. 
Sediment  carried  into  ocean,   46. 
Selective   absorption   by  soils,   nature   of, 
269. 

types    of,    269. 
Selection  of  a  commercial  fertilizer,   fac- 
tors  to   consider,    482. 
Sheet  erosion  and  its  control,   205. 
Size  of  colloidal  particles,  128. 
Slope,  influence  on  soil  temperature,   229. 

influence    upon    absorption    of    solar 
insolation,   229. 
Sod,    influence    on    nitrate    accumulation, 

427. 
Sodium    chloride    as    a    soil    amendment, 
380. 

presence  of  in  alkali,   333. 
Sodium  nitrate,  changes  in  soil,  448. 

character  of,  448. 

composition  of,  448. 

retention  of  by  soils,  321. 

origin  of,   448. 

source  of,   448. 
Soils,   absorption  by,   263. 

absorptive  capacity  of,  266. 

acid  nature  of,  345. 

acquisition  of  nitrogen  by,  428. 

addition   of    sulfur   to   by   precipita- 
tion.    469. 

seolian,   61. 

alkali,   328. 

alluvial,    46. 

amendments  used  on,   363. 

ammonification   in,   412. 

amounts  of  capillary  water  in,  166. 

amount  of  gravity  water  in,  177. 

available  water   of,    198. 

average    composition     of,     12. 

bulk  analysis  of,    311. 

capacity  of  to  retain  nitrates,  321. 

capillary  capacity  for  water,   163. 

capillary  movement  of  water  in,  168. 

capillary   water    of,    159. 

cause  of  acid  condition  of,  351. 

changes  of  lime  in,   369. 

chemical    analysis    of    water    extract 
from,    320. 

colluvial,   45. 

color  of,    36. 


584 


INDEX  OF  SUBJECT  MATTER 


Soils,  composition  of  air  in,  247. 

conductivity  coefficients  of,   236. 
conditions,      effect     on     nitrification, 

418. 
control    of    air    in,    261. 
control  of  alkali  in,  343. 
control    of    erosion,    205. 
cumulose,    42. 
defined,     2. 

diseases,    control    of,    397. 
diseases,  nature  of,   396. 
dynamic   nature   of,   3. 
eradication  of  alkali  from,  341. 
erosion    of   by   water,    204. 
fertility    evaluation    of    by    chemical 

analysis,    323. 
formation    of,    16. 
forms  of  water  in,  152. 
functions  of  water  in,   184. 
general  composition  of,  2. 
geological   classification  of,    38. 
glacial,    54. 

granulation    of,     defined,     139. 
handling  of  alkali  soils,  340. 
heat  conduction   in,    234. 
heat  convection  in,  238. 
hygroscopic   water  of,    152. 
importance  of  absorption  by,  273. 
influence  of  earth  worms  on,  385. 
insolation    received    by,    225. 
lacustrine,   58. 
losses   of   water   from,    202. 
management,     practical     factors     of, 

560. 
marine,  50. 
method     of     moisture     determination 

on,    161. 
moisture  data  of,  200. 
movement  of  air  in,  258. 
movement   of  gravity  water   in,    175. 
mulch   on,   218. 
names  in  common  use,  82. 
names,  origin  and  meaning,  80. 
nitrification    in,     416. 
nitrogen    content    of,    118. 
partial   analysis   of,    315. 
partial    analysis    with    strong    acids, 

316. 
partial     analysis    with     weak     acids, 

317. 
particles    of,     67. 
plasticity  of,   140. 


Soils,  pore  space  of,   93. 

practical   management  of,    560. 

productivity    of    as    related    to    soil 
solution,    287. 

puddling    of,    141. 

reaction,  importance  of,  345. 

reaction,  types  of,  845. 

reduction    of    nitrates    in,    424. 

residual,    38. 

sampling    of,    311. 

series    defined,    86. 

specific   gravity    of,    88. 

specific    heat    data,    232. 

sulfofying    power    of,    405. 

survey  classification  of,   85. 

tests  for  acidity  in,  354. 

thermal    movement    of    moisture    in, 
182. 

the  solution  of,  275. 

tilth,   defined,    149. 

toxins  of   organic  nature,    108. 

type   defined.    86. 

weathering,    importance    of,    37. 

weight   of,    data,    93. 

wilting  coefficient,   197. 
Soil  acidity,  active  toxic  bases,  346. 

as   influenced  by  absorption,   274. 

causes  of  development,   352. 

causes  of  harmful  effects,   346. 

expression  of  by   ph  values,   350. 

general    nature    of,    345. 

influence  of  absorption  on,  352. 

influence  of  fertilizers  on,   353. 

influence   of   leaching   on,    352. 

influence  on  bacteria,  395. 

influence    on     nitrification,     421. 

lack   of  calcium   in   relation  to,   348. 

lack   of   nutrients   theory,    348. 

lime  requirements,   determination   of, 
355. 

litmus    paper    test   for,    358. 

present  status  of  question,  349. 

relation  of   iron   to,    347. 

relation   of  manganese  to,   347. 

resume  of,   360. 

tests    for,    354. 

theory,    aluminum,    347. 

theory,    hydrogen    ion,    346. 

tolerance  of  plants  to,   353. 

Truog    test    for,    358. 

types  of  tests,   355. 

zinc  sulfide  test  for,  358. 


INDEX  OF  SUBJECT  MATTER 


585 


Soil    air,    carbon    dioxide    of,    250. 

general    characteristics,    247. 

composition    data,    248,    250. 

control     of,     261. 

general   composition   of,    247. 

movement    of,    258. 

resume     of,     262. 

types    of,     249. 

volume   of,    257. 
Soil    amendments,    forms  of   lime,    363. 

organic  matter  important  as,  124. 
Soil  analysis,  alluvial  and  upland,  49. 

arid  and  humid  soils,  31. 

determination      of     organic     matter, 
112. 

glacial    soils,    57. 

granite    soil,     33. 

good  and  poor  soils,  326. 

humus    determination    of,    115. 

humus    in    California    soils,    120. 

humus   in   Nebraska   soils,    120. 

lime  requirement  of  soil,  355. 

limestone    soil,     33. 

loess    soils,    63. 

marine    soils,    52. 

mechanical,   67. 

nitrogen  in  California  soils,    120. 

nitrogen    in    Nebraska   soils,    120. 

nitrogen    in    soils    of    United    States, 
118. 

organic    matter    in    Nebraska    loess, 
,    120. 

organic    matter    in    Minnesota    soils, 
119. 

organic    matter    in    soils    of    United 
States,     117. 

peat   and    muck,    44. 

residual  soils,  41,  52,  57. 
Soil  class,   discussion  of,   79. 

determination     from     a     mechanical 
analysis,    84. 

practical   determination  of,   83. 
Soil    colloids,    absorption    by,    265. 

as  influenced  by  plant  roots,  297. 

generation  of,  132. 

importance    of,    135. 

influence  of,  135. 

resume  of,    138. 
Soil    color,    cause    of,    36. 

significance    of,    36. 
Soil  erosion  and  its  control,  204. 

types    of,    205. 


Soil    exhaustion,    discussion    of,    309. 

possibility  of,    308. 

time   for,    309. 
Soil  extraction,  a  method  of  studying  the 

soil  solution,  279. 
Soil   fertility,   defined,   554. 

effect  on  transpiration,    192. 

factors  involved  in  maintenance,  554. 

importance  of  nitrification  to,  423. 

influence   of   plants  and   animals   on, 
23. 

maintenance  program  of,   560. 

relation  of  sulfur  to,  468. 

sources  of  knowledge,  554. 
Soil  formation,   forces  of,    16. 

general  statement  of,   29. 

glacial   action,    18. 

influence  of  carbonation,  26. 

influence   of   climate,    30. 

influence   of   hydration,    26. 

influence  of   solution,   27. 

oxidation   and   deoxidation,   24. 

processes    classified,    16. 

special  cases  of,  32. 

temperature  changes,    21. 

water   action,    17,    19. 
Soil    heat,    importance    of,    223. 

influence  of  on  the  soil,  224. 

loss  of  by  conduction,   240. 

loss  of  by  evaporation,  240. 

loss  of  by  radiation,  240. 

transfer    of,    238. 
Soil  humus,    determination   of,   115. 
Soil    minerals,     importance    of,     6. 

list    of,    5. 
Soil  moisture,  conservation  of,  219. 

data    of,    179. 

effect   on   conductivity   of   heat,    236. 

effect  on  heat  capacity,    233. 

effect  on  transpiration,  191. 

importance    of    amount    in    plowing, 
146. 

influence    of    on    the    soil    solution, 
286. 

optimum  for  efficient  tillage,  150. 

optimum    for    plants,    200. 

relation    of    to    granulation,    142. 
Soil    mulch    and    moisture    conservation, 
221. 

relation    of    to    capillary    movement, 
175. 

use   of,    218. 


586 


INDEX  OF  SUBJECT  MATTER 


Soil  organic  matter,  amount  of  in 

Minnesota    soils,    119. 
amount  of  in  soils  of  United  States, 

117. 
general   nature,    7. 
importance    of,     121. 
maintenance  of,  122. 
resume  of,    126. 
source    and    character    of,    99. 
Soil    organisms,    and    the   free-fixation   of 

nitrogen,     431. 
benefits  of,  397. 
general  methods  of  study,  399. 
groups  of,   384. 

influence  in  nitrate  assimilation,  426. 
influence  of  alkali  on,   335. 
injurious  to  higher  plants,  396. 
macro-animal  forms,   384. 
macro-plant    forms,    385. 
micro-animal    forms,    386. 
micro-plant     forms,     388. 
resume     of,     440. 
Soil  particles,  character  as  determined  by 

size,     69. 
classification    of,    67. 
minerological     character,     75. 
number   of,   95. 
surface   of,    97. 
Soil  separates,  chemical  and  minerological 

characters,   75. 
chemical  composition  of,    78,   79. 
physical   characters   of,    73. 
sizes    of,    67. 
specific    gravity    of,    89. 
Soil  solution,  as  studied  by  aqueous  ex- 
traction,   279. 
as  studied  by  depression  of  freezing 

point,     280. 
composition  data  of,   283,   288. 
concentration     data     of,     282,     285, 

286. 
general  character  of,  275. 
influence  of  crop  on,  284. 
influence  of  miscellaneous  factors  on 

286. 
influence  of  season  on,   283. 
methods    of    study,    277. 
qualitative  composition  of,  280. 
quantitative  composition  of,    282.  . 
relation  to  absorption,  276. 
relation  to  productivity,   287. 
summary    of,    288. 


Soil    structure,    ideal,    88. 

nature    of,    87. 

types  of,   139. 
Soil   temperature,    control   of,    244. 

data  of,    243. 

influence    of    slope,    229. 

variations   of,    242. 
Soil  water,  availability  of.    198. 

diagram  of  forms,  199. 

effect  on  air  movement,   258. 

effect  on   specific  heat  of  soils,   233. 

form  of  molecule,  28. 

forms    of,    151. 

function  to  plants,  184. 

general   characteristics  of,   152. 

influence  on  plants,   186. 

loss  by  evaporation,  216 

loss  by  percolation,   206. 

loss  by  percolation  at  Cornell,  209. 

loss    by    percolation    at    Rothamsted, 
207. 

modes    of    loss,    202. 

methods    of    expressing,     156 

run-off    losses,    203. 

summary    of    control,    221. 

thermal  movement  of,    182. 
Soluble  matter  carried  into  ocean,  46. 
Soluble  salts   in   soil,   influence  on   nitri- 
fication,    421. 
Solubility    of    nutrients   as   influenced    by 

carbon   dioxide,    255. 
Solution,  loss  of  nutrients  because  of,  23. 

importance  of  in  soil  formation,    27. 

relation   of   to   soil   productivity,    28. 
Specific  gravity  of  minerals,  89. 
Specific  gravity  of   soils,   defined,   88. 

determination  of,    90. 
Specific  gravity  of  soil  separates,   89. 
Specific    heat,    data    on    soils,    232. 

defined,    231. 
Specific    heat    of    soil,    231. 

factors     affecting,     232. 
Stages    in    the    decay   of    green    manures, 

542. 
Stassfurt    salts,     chlorides    and    sulfates, 
463. 

kainit,    463. 

silvinit,    463. 
Stone   drains,   construction  of,    212. 
Straw,     influence     on     nitrate     reduction, 

425. 
Streams,   soil   formation  by,   46. 


INDEX  OF  SUBJECT  MATTER 


587 


Structure     of     soil,     effect     on     capillary 
capacity,    164. 

effect  on  capillary  movement,  174. 

effect    on    gravity    water,    176. 

effect  on  heat  conductivity,    236. 

ideal    condition,    88. 

influence   on  optimum   water,    201. 

nature   of,    87. 

summary    of,    149. 

types  of,  139. 
Substitutions   of   bases  in   soils,    270. 
Sulfate   sulfur   as   a   fertilizer,    468. 
Sulfofication,    effect    of    lime    on,    405. 

factors  influencing,   405. 

influence   on   carbon    dioxide   produc- 
tion,   255. 

determination  of,  405. 

reactions    of,    403. 

relation    of    to    mineral    cycle,    408. 
Sulfur,  amount  added  to  soil  in  precipi- 
tation,    469. 

amount   in   soils,    13. 

as   a   fertilizer,    467. 

experiments     with     as     a     fertilizer, 
467. 

forms  of  in  soil,  11. 

how  lost  from  soil,   404. 

importance   of   in   soil   fertility,    470. 

loss  of  from  Cornell  soils,  307,   404. 

loss  of  from  soil,  555. 

natural    addition    to    soil,    557. 

oxidation   of   in    soils,    403. 

possible    deficiency    in    arable    soils, 
468. 

pounds    removed    by    various    crops, 
468. 

sources  of   in   soils,    403. 

use    in    composting,    406. 

use  of  as  a  sulfate,   468. 
Sulfur   composts,    406. 
Sulfur  cycle  of  soil,  losses  of  sulfur  from, 
404. 

sources  of  sulfur,  403. 

sulfofication,    403. 
Sulfurous  acid,  relation  to  mineral  cycle, 

408. 
Superfluous  water,    198. 
Surface   of   soil   particles,    calculation   of, 
97. 

importance   of,    97. 
Surface  tension,   defined,   160. 

effect    on    capillarity,    170. 


Surface   tension,    force   of,    160. 

relation  to  capillary  movement,   169. 
Synergism,    relation   of   to   plant   absorp- 
tion,  300. 

relation  of  to  soil  acidity,  349. 

nature     of,     349. 
Systems   of    applying   fertilizers,    496. 

Tankage   changes   in   soil,    445. 

character    of,    445. 

composition    of,    445. 

source    of,    445. 
Temperature  of  soil,  control  of,  244. 

data    of,    243. 

effect  of  change  on  soil  air,  259. 

effect  on  capillary  capacity,   163. 

effect  on  gravity  water,    176. 

importance  in  soil  formation,  21. 

influence  of  decay  on,   239. 

influence    of    slope,    230. 

influence   on   bacteria,    394. 

influence    on    hygroscopic    coefficient, 
158. 

influence  on  nitrification,   420. 

variations   of,    242. 
Temperatures  for  crop  growth,  224. 

for  germination  of   seeds,    224. 
Terracing,     205. 
Texture    of    soil,    definition    of,    66. 

effect  on  absorption,   267. 

effect    on    capillary    capacity,    164. 

effect  on  capillary  movement,   173. 

effect    on    gravity    water,    176. 

effect  on  heat  conduction,   236. 

effect    on    specific    heat,     232. 

influence     on     moisture     equivalent, 
168. 
Thermal    movement    of    soil    water,    na- 
ture   of,    182. 

relation  to  evaporation,    182. 
Tile   drains,    depth    and   interval   of,    214. 

effective  grade  for,   214. 

functions    of,    212. 

outlets   of,   214. 

size    of    tile,    213. 

study   of   drainage  water   from,    180. 

systems,    212. 

table  for  determination  of  size,    214. 
Tillage,    influence   on   granulation,    144. 

influence  on  soil  solution,   286. 

killing  of  weeds  by,   219. 
Tilth  of  the  soil,  defined,  149. 


588 


INDEX  OF  SUBJECT  MATTER 


Time,    influence    on    absorption    by    soils, 
269. 

Time   of    applying   fertilizers,    495. 

Tolerance  of   plants   to   soil   acidity,   353. 

Tramping,     influence     on     farm     manure, 
524. 

Transpiration,   factors  affecting,   188. 

Transpiration    ratio,    defined,     187. 
determination    of,     187. 
of    different    crops,    data,    189. 

Transported  soil,  explanation  of  term,  28. 

Truog  test  for  soil  acidity,  358. 

Types  of  farming,  influence  on  the  main- 
tenance of  fertility,   558. 

Urea,    ammonification    of,    414. 

decomposition  of  in  manure,   509. 

production    of    from    calcium    cyana- 
mid,    250. 
Unavailable  water  in  soil,    198. 
Unmixed  fertilizers,  purchase  of,   484. 

use   of,    484. 
Utilization    of     ammonium     in    salts    by 
higher   plants,    450. 

of     organic     compounds    by     plants, 


Variability    of    farm    manure,    506. 
Value  of  farm  manure,  agricultural,   513. 

commercial,     512. 
Vegetables,   fertilizer  formulae  for,   491. 
Vegetation,  resistant  to  alkali,  340. 
Veitch    method   of    determining    the   lime 
requirement   of   soils,    356. 

procedure,   356. 

value   of,    357. 
Viscosity,  effect  on  capillarity,  170. 
Volcanic    dust,    as    soil,    65. 
Volume   of   soil   air,    257. 

calculation  of,   258. 
Volume  weight,   determination  of,   91. 

relation  to  specific  gravity,  94. 
Volume  weight  of  soils,  data,  93. 

explanation    of,     91. 
Water,    alkali   in   river   water,    332. 

availability  of  to  plants,  198. 

deposition  of   sediment  by,   46. 

diagrams  of  forms  in  soil,  199. 

effect  on  rocks  by  freezing,  23. 

erosive    effects    of,    204. 

function  of  to  plants,  184. 

in   farm   manure,    501. 


Water,   influence  on  concentration  of  soil 
solution,    286. 
influence  on  plants,    186. 
intake   of    by    plants,    289. 
loss    of    from    soil,    202. 
loss    of    from     soil     by    percolation, 

206. 
loss    of    from    soil    by    evaporation, 

216. 
mechanical  action  of,  17. 
methods   of    expression    in    soil,    156. 
movement  in  soil,   168,   175,  182. 
movement     in     soil     in     relation     to 

plants,    193. 
production   of   hydration   by   in   soil, 

27. 
relation    of   to   granulation,    142. 
required  to  mature  a  crop,  193. 
use    of    alkali    water    in    irrigation, 
333. 
Water    requirements    of     plants,     factors 
affecting,     188. 
investigations    of,     189. 
nature  of,  187. 
Water  slaked  lime,  364. 
Water    soluble    phosphoric    acid,    defined, 

456. 
Weathering,  character  of  in  arid  regions, 
30.  -   • 

character  of  in  humid  regions,   30. 
defined,   16. 
losses    due   to,    33. 
of  granite,   33. 
of    limestone,    33. 
of  soil,   practical  relations  of,   37. 
relation  of  to  alkali,  331. 
Weeds,    killing    of,    219. 
Weight    of    soils,    data    of,    93. 
Wilting,   cause  of,    194. 

explanation    of,    194. 
Wilting  coefficient,   calculation  of,   198. 
determination  of,  196. 
effect  of  texture  on,   196. 
explained,    195. 
for    different    soils,    197. 
Wind  in  soil  formation,  19. 
Wool    and    hair    waste,    composition    of, 
446. 

Zinc-sulfide  test,    criticism  of,   360. 

for  soil  acidity,  358. 
Zeolites,  not  present  in  soil,   265. 


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